Protestants such as myself use the Masoretic Text for the translation of our Bibles. However, the Eastern Orthodox Church uses the Septuagint, a Greek translation we get primarily from the three large codices: Vaticanus, Sinaiticus, and Alexandrinus. How can I explain to someone why the Septuagint is good, but has problems (aside from the fact that Deuterocanonicals may have been included)? What’s so much better about the Masoretic Text pertaining to the Dead Sea Scrolls?
First of all, I am not sure I agree that it is as simple as the claim that Protestants use the Masoretic Text and Orthodox use the Septuagint. There may be a grain of truth in what you say, but the distinction is not nearly as clear cut as you say. The fact is that 99% of all Protestants use a translation of the Bible and certainly do not read either Hebrew (Masoretic Text) or Greek (Septuagint). The same is true of Eastern Orthodox Christians. Virtually all of us use translations. As far as I know, our Orthodox Christian friends who read the Bible use more or less the same translations as we do. My sister is an Orthodox nun. I believe she uses a standard English translation—not one done by the Orthodox Church. So, for virtually all Protestant and Orthodox believers, they read neither the Masoretic nor the Septuagint. They use translations by reputable scholars and all scholars that I know of use both the Masoretic and the Septuagint texts in attempts at arriving at the best English translation. To the extent this is true, your question becomes somewhat of a moot point.
But perhaps you are talking about Eastern Orthodox priests or scholars, or perhaps you are talking about the Old Testament used by the Eastern Church many centuries ago. I am not sure what you are referring to when you say that the Eastern Orthodox use the Septuagint. I am not aware of any Eastern Orthodox believers who prefer use of the Septuagint over the Masoretic today. Here is the deal: In the very early church, when there was no separate Roman Catholic and Eastern Orthodox Churches, one thousand years before there even was such a thing as a Protestant, the Christian Church primarily read the Old Testament from the Septuagint, not the Masoretic Text. This was for two reasons. First of all, the Church was composed mostly of Greek speakers at that time, so, naturally, they read the Bible in a Greek translation, which was the Septuagint. Very very few of the early church read Hebrew, so, naturally, they used a translation and the Septuagint was the only Greek translation available in the early church. The disciples could not read the Masoretic Text even if they wanted to. Another reason the early church did not use the Masoretic Hebrew Bible is that it literally did not even exist at that time. The Hebrew Masoretic Text was produced in the eighth and ninth centuries by Jews in an attempt to create a standard Hebrew text. There was literally no Masoretic Text available in the early Christian Church when the Eastern Church was using primarily the Septuagint! We can assume that the primitive Church had access to Hebrew Old Testaments, but we cannot be sure exactly what the texts at that time were like.
As for the relative advantages and disadvantages, the Masoretic Text has some textual issues because it was produced from Hebrew texts from the 8th and 9th centuries. The Septuagint Text has a different problem, which is that it is not in the original language, but the advantage of the Septuagint for accuracy is that it is a translation made about 250 years BC, so it reflects relatively better Hebrew text. Good scholars who translate the Old Testament into English take into account both the Masoretic and the Septuagint texts, as well as the Dead Sea Scrolls. In fact, if you look in the margin of your Bible, they will often note which of the versions is being used, as well as giving the translation of the other texts in the margin.
Here are the facts, as I understand them. Virtually all in the English speaking world use neither the Masoretic nor the Septuagint. We read translations. All good translations take both the Masoretic and the Septuagint (as well as the Dead Sea Scrolls) into account in a more or less balanced way. This is true of Protestants, Catholics and Eastern Orthodox. It is also true of people like myself who are Christians, but who identify as neither Protestant, Orthodox or Catholic, but simply as Christians. It might be true that some Eastern Orthodox scholars even today prefer to look to the Septuagint over the Masoretic because of the tradition handed down from the early church, but it is hard to see how this could have any significant impact on what is believed by the average Orthodox or Protestant believer.
You say that you use the Masoretic Text for the translation of your Bible. I would challenge that statement. I do not know what translation you use, but as far as I know all important English translations use both the Masoretic and the Septuagint Texts in making their translations. There is no modern English translation which relies solely on the Masoretic Text, although really old translations such as the King James did rely quite largely on the Masoretic. I just pulled out my trusty NIV and opened to a random page. I found that in Isaiah 10:27 it gives the translation of both the Septuagint and the Masoretic Texts for this verse. The NIV translators often preferred the Septuagint, but more often prefer the Masoretic Text, but both are used, depending on the context and the history of the text.
As for why the Septuagint is good, one reason that this translation is helpful is that it is a translation from Hebrew made in about 250 BC. It is kind of like a “snapshot” of what the Hebrew text looked like over a thousand years before the Jews created the Masoretic Text. The Septuagint (for example in the Alexandrinus or Vaticanus texts) is older than the Masoretic. The oldest extant Masoretic Text is from the late 9th century. In any case, the differences, due to copying errors and slips of the pen, are quite minor and, generally, the Masoretic Text and the Septuagint will produce the identical translation into English. Yes, it is true that there are some advantages to the Septuagint, but the majority of our translations do come from the Hebrew Masoretic Text, even though it is from a thousand years after, simply because it is in Hebrew!!!
Whether the Septuagint included the Apocrypha depends on what your definition of the Septuagint is. The Jews (who also use the Septuagint at times in their scholarly work) do not now, nor did they ever consider the Apocrypha as part of the Septuagint. This is a question of canon, not a question of text or translation. The fact is that the Jews never accepted books such as Ecclesiasticus or Tobit or Judith to be part of their canon, but the early church did use these books, as they considered them part of their Bibles. You and I consider the use of the Apocrypha by the early church a mistake, but this has no effect on whether the Masoretic or the Septuagint are better texts to understand the Old Testament. For the sake of simplicity, let us keep canon issues (what books belong in the Bible) separate from textual issues.
You ask, “What is so much better about the Masoretic Text pertaining to the Dead Sea Scrolls?” Sorry, but this question does not really make sense as written. The Dead Sea Scrolls are one thing, the Masoretic Text is another. The Dead Sea Scrolls are a third source of material for trying to produce the best Hebrew text. The fact is that often the Dead Sea Scrolls are closer to the Septuagint and often they are closer to the Masoretic Text. The two texts of Isaiah found in the DSS are an example of this dichotomy. One is a bit closer to the Septuagint. The other is closer to the Masoretic. The fact is that all three are very similar and the final translation into English is only very little affected by the slight differences. For many scholars the Dead Sea Scrolls carry higher weight than either the Masoretic or the Septuagint, because they are a truly ancient Hebrew text. Logically, one would expect a Hebrew text from 150 BC to be, on average, closer to the original than a Hebrew text from 800 AD. This is why the DSS are favored in translation. However, a wise scholar will use all three. I am confident that the English translation you read from uses all three.
In the end, this is a fascinating topic and we, as English speakers, are fortunate to have so many sources for producing good translations of the Old Testament into English, but for everyday use of the Bible, the distinctions between the Masoretic, the Septuagint and the DSS is not a significant issue in how we live out our Christianity.