Okay folks. Let's look at the facts, and draw conclusions from them.
Manuscripts still in existence include:
The Dead Sea Scrolls (from 200 BC to 70 AD)
The Geniza Fragments (Written in Hebrew and Aramaic. 400 AD,)
The Ben Asher Manuscripts (Masoretic Hebrew from 700 to 950 AD. They include the following:
The Aleppo Codex - 950 AD, much of it was destroyed in 1947.
Codex Leningradenis - AD 1008 - the complete Old Testament.)
In 400 BC the OT was translated into Aramaic, and called the Aramaic Targums.
In 250 BC the OT was translated into Greek. That was the Spetuagint, and is sometimes called the LXX. New Testament writers often referred to the LXX when they referred to OT scriptures.
Today we still have the Chestery Beatty Papyri and the Codex Vaticanus and the Codex Sinaiticus. These are all parts of the Septuagint.
There over 5,600 Greek Manuscripts of the New Testament in existence.
In 180 AD people began copying the NT fro mGreek to Latin. In 380 AD St. Jerome translated the Latin Vulgate. Jerome translated from Hebrew in the OT, and from Greek in the NT. Jerome's bible was the most accepted and commonly used bible until the 1500's. The Catholic's still look at this translation as the 'real' bible.
In 1380 AD John Wycliff translated from the Latin Vulgate into english.
In 1522 AD the Polyglot Bible was published. The OT was in Hebrew, Aramaic, Greek and Latin. The NT was in Latin and Greek. Tyndale used the Polyglot to translate the OT into English. He died before he could complete it.
In 1611 AD translators for King James used the Textus Receptus to translate the bible into English.
In 1968 AD the United Bible Societies 4th Edition of the Greek New Testament came out. This translation was made from the oldest existing Greek manuscripts (175 AD). This is the what the translators of the NIV used to translate from Greek.
In 1971 AD the NASV was published. It used Hebrew and Greek manuscripts that were older than previously used manuscripts, and which were not available for translation when the KJV was created. It is almost a word for word translation from the Greek.
In 1983 AD the NIV was published using the oldest manuscripts available in the original tongues. It was a "thought for thought" translation. To see the difference....
Hebrews 9:25 NASV - The high priest enters the holy place year by year with blood not his own.
Hebrews 9:25 NIV - The high priest enters the most holy place year by year with blood not its own."
The difference, in meaning, is that the NIV translators looked at the fact that on the Day of Atonement high priests performed their work in the "Most holy place" (Holy of Holies) in the temple. The word "hagios" translates to "holy place," but the reference, in context in Greek, is clearly to the Holy of Holies, so the translators put the meaning of the Greek back into the translation.
This is a common problem, and hence debate, when translating.
Word for word translation of "I love you," from english to french would be "Je t'amour," however to a french man, Je T'amour is sexual in meaning. Je t'aime, is word for word, "I like you," but to the french it means what we mean when we say, "I love you."
The KJV was translated from Erasmus' printed Greek New Testament which used 5 greek manuscripts that dated to 1,100 AD.
The NASV and NIV used the UBS's printing of the New Testament in Greek in 1968. This addition looked at over 5,600 greek manuscripts written in the Alexandrian style. These manuscripts date back to 175 and 350 AD, or to put it in context, to within 100 years of the time Christ died for our sins. The early churh of around 97 to 350 AD (note, 97 would only be 64 years after Christ went to the cross), used Alexandrian texts when they quoted the NT.
So, when you look at the NIV and or NASV and say it "adds" to the bible, understand, they actually use older biblical manuscripts than King James used.
Now, I love the King James Bible, and I don't really see any differences in meaning between it and the NIV or NASV, but then again, I have a habit of going back to Greek and Hebrew to clarify the meaning of scripture, so what might "seem" different on the surface, is not different to me once I see the original Greek and Hebrew. I think the KJV is much more poetic, and therefore more easy to memorize. However, it is, at times, more difficult to teach because of the language.