I was viewing your site on the net, and the question came as to why you would use a translation which has been tampered with and seems to be corrupt, leaving out complete verses and part of verses. The New International Version and the New World translation seem to be almost identical in this respect.
I don't use the New World version, but it is true that I do occasionally quote from the NIV (always identified as such). Actually, I do have quite a number of problems with and reservations about the NIV, and am "in print" on the subject: see "Read your Bible: Protection against Cults". Like many translations, the NIV has its good and bad points (although there are some versions like "The Living Bible" which have mainly bad points). In my own studies I use the original Greek, Hebrew and Aramaic.
In my view no theological weight whatsoever should be placed on any passage purely from an English translation done by someone else. The process of translation is a very difficult and time-consuming one. I have translated and re-translated many verses of the Bible over many years and always seem to be fine tuning them whenever I find "a better why to put" whatever is in the original language (and as my understanding of the theology, tone and circumstances behind them deepens). There is no perfect translation. A perfect translation would require perfect knowledge of ancient Greek, Hebrew and Aramaic , of the historical and cultural background, a perfect knowledge of what the Bible really means in its theological entirety, and a perfect, artistic ability to translate into another tongue, not only conveying the meaning perfectly, but also the tone, style and emotional force as well. In other words, there can't be a perfect translation, and as language (our language) is always changing, even there ever were, it would be out of date as soon as it was completed. That is not to say that we cannot get what God intended us to get out of the Bible - Ichthys is a ministry dedicated to just that, and everything offered here is based upon painstaking work in the Greek, Hebrew and Aramaic original.
That said, there are times when, in my opinion, another version has gone far toward capturing the essence of a passage in such a powerful way that it is difficult to improve upon. In such cases, it is my practice to quote from that version (NIV, KJV, NASB, whatever). But as I say, this is always indicated in the text, next to the verse citation. Wherever you see passages translated at Ichthys which are not marked in this fashion, it means the translation is my own.
The NIV does not, to my knowledge, leave out passages that should be included (the KJV, done at a time when the best biblical manuscripts had still not come to light, does include verses that ought not be included, because they are not part of what the original authors wrote: the longer ending of Mark is the classic example). On the other hand, it is true that the NIV in particular includes things that aren't really there (it's very "interpretative" - only a good thing when the interpretation happens to be right). But in this it is not alone - all translations do add things to some extent (on some level it is a necessary element of translation: Greek, for example, loves to leave out the verb "to be"). The only way to avoid problems on this score entirely would be to stay away from reading English translations of the Bible altogether, and that would be a terrible mistake. The problem of mis-translation can be compensated for by doing some of the things I suggest in the study linked and cited above ("Read Your Bible"). One thing that is relatively easy to do is to read several translations at once, and to double check from another translation whenever something in the text you are reading seems "off" or "too good to be true". Perhaps the most important thing is to avail oneself of good, solid Bible teaching, teaching which is based on meticulous exegesis from the original language and is dedicated to systematically answering questions about what the Bible means. Put these together with prayer, a sanctified walk, and a life of seeking to serve Him, and one has a recipe for seeking and being led to the truth which no imperfection of any one or any number of translations will be able to hinder or prevent.
Hope this gets to your question. You might also see these links:
Only-Begotten, Mother-of-God, On-this-Rock: Why English-only Approaches to Bible Interpretation are Dangerous.
The Book of Job and Biblical Interpretation.
Hermeneutics (in CT 1)
How can we know whose interpretation of the Bible is correct (part 1)?
How can we know whose interpretation of the Bible is correct (part 2)?
Bible Interpretation: Interlinears, Academics, Versions et al.
In Him who is the Truth, our Lord Jesus Christ.