If we don't have a perfect Bible we can trust every word, then we lose against the world.
(Bob is saved, but uses a modern version.)
Bob: "Hey world, can I show you from my Bible what Jesus did for you?"
World: "Your Bible? Is it trustworthy? You don't really believe that book do you?"
Bob: "Well, I believe most of it to be true."
World: "If you don't believe all of it, then why believe what it says about this Jesus?"
Bob: "Oh, because that part is true."
World: "So I'm supposed to trust what you say about a book you don't even believe is 100% trustworthy? No thanks."
Bob would lose the argument every time unless he has a final authority to appeal to. I'm not saying that one cannot get saved reading a modern version or a tract for that matter. The problem is facing a world with something we don't trust 100% to be the words of God.