Just so as you don't feel ignored, I thought I'd post. Unfortunately, though, I don't have any particular opinion on the matter and have never made such an accusation. I have read the Boook of Mormon but it left me completely cold. I don't believe it's Scripture and wouldn't accept it alongside the Bible, but I don't believe the canon was ever closed as such and, in principle, other texts could (though I doubt they will) be canonised in future. All of my disputes with LDS Christians are doctrinal, however, rather than revolving around the question of what is or is not Scripture - probably because I'm not a sola scripturalist in the first place. Give it time, though, some other non-LDS Christian who does have more of an opinion on this will probably post here eventually.
James