Circle hooks probably aren't as necessary for LMB since they do, in fact, have large mouths. However, for releasing bluegills, yellow perch, red-ear sunfish and other fish with small mouths, they really do work well. Also for catfish that you wish to release.
As many people have said, using circle hooks requires a very different technique than do other hook shapes. I think that they are extremely useful for kids and other inexperienced fishermen.
