I understand that that verse was written for the book of Revelation, however the principle remains the same. Just like in the New Testament Jesus says he is coming to nail the old law to the cross, meaning the ten commandments, because we are to follow the new testament. The principles of the old testament are there, we know we should not kill, however in the new testament we take it one step further and are told not to hate our brother, because that is like murder. The bible as we are told in Timothy is inspired by God...now how can we imperfect humans believe something that is inspired by God be added to or deleted by us. Just because we want to improve on it? The bible was written in the perfect form by God, you can not improve it. Thus we should follow the principle of that verse in Revelation not to add to or take away from.
Citation on that?
So what about the protestant reformation where entire books of the Bible were removed?
Good question. Ask Martin Luther - he wanted to delete Hebrews and the Letter of James, too.
But seriously, simply by saying that "of course" a verse that, in its context, pertains to the Book of Revelation, "really" applies to all the texts identified as "inspired" over the next couple of hundred years ... when you do that, you're adding interpretation to the literal meaning of the text, and declaring the interpretation to be "perfect."
Pardon the overuse of quotation marks ... I'm having "one of those days."