The Bible wasn't even compiled for 2-3 centuries after these books and epistles were written. Revelations wasn't even written last in the canon. If you apply that statement to the entire Bible, it means that we're not allowed to read anything written by James, Paul, or even the other books written by John.
If you go by your interpretation, then you must also throw out the entire bible after Deuteronomy 4:2-3, since the Lord is commanding pretty much the same thing, not to add to or take away from His word.
Most bible scholars will tell you to be careful how you use that reference from Revelations. And I'd not try to push that agenda here, especially here on a political thread.
Against my better judgment, I will offer one bit of scriptural advice, something I read again today at Church that reminded me a lot of the absurd amount of Mormon bashing that goes on here at FR, and why I think it is so misplaced.
1 John 4:1-3
1 Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world.
2 Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God:
And you certainly don't need to wonder where we stand on the subject of Christ coming in the flesh. It's central to our teachings.
http://scriptures.lds.org/en/gs/j/22
Make it a great day!
If you truly knew your Bible, you'd know that the last book of Scripture is in fact called "Revelation", as in "The Revelation of St. John The Divine."
Good night.