A very true statement. But it also needs to be said that a person can't be a Mormon unless he accepts Joseph Smith as God's prophet. That is the primary distinction between Mormonism and Christianity. The focus of one's belief.
Sure you can be a Mormon if you don't believe. I agree there wouldn't be much point, though.
But I could make a similar claim about any Christian. You wouldn't be much of a Christian if you didn't believe that Moses, Abraham and Jacob were prophets, or that Peter and John were Apostles. Sure, you could still believe in Christ but you'd be throwing out a lot of the Bible.
Believing in a true prophet says nothing about one's focus on Christ. Well, I take that back. It says a lot about one's focus on Christ if you believe in the prophets He has sent rather than "stoning" them.