Another thing I've noticed about guards is I get more crumbs in the eye using a grinder with a guard. You know where they're going without it, but they go everywhere when they hit the edge of the guard, come in right behind the glasses.
I have had one 9" wheel come apart on me, but it was my fault- I'd dropped the grinder and thought it didn't damage the wheel. It came apart pretty good, cut my leg, but no big deal.
John