There are other verses with Jesus himself saying to not "add" to scripture. And that doesn't mean to not write more. But he was indicating that what was written down should line up with previous scripture.
My claim is that things in the Book of Mormon conflict. It adds new stuff. New ideas. Not extrapolations of old ideas. It also poses Jesus coming again, and Jesus said he would not come again until the end.
And just for the record, I'm not saying that Mormons are not Christians, or at least I'm not saying that none of them are.
The only thing that really matters, in the long run, is that you know Christ, and that Christ knows you. Christ is your advocate. The only way to the Father is through Christ, and that is what that really boils down to. Not whether you think saying His name over and over again is important. Not how much you go to church, or even if you do or have at all. Not even whether you actually know Christ by name, or whether you have actually come to hear the message of the Bible. It's just easier and more guaranteed if you do or have.