You can't "waste" an HM, it's infinitely reusable. Furthermore, although unlikely, it is possible to use vs-grade 'mans in gameplay. All you have to do is either EV train ahead of time (and pray someone can trade you the Power- items), or invest in a lot of stat-reducing berries when you're done.
Inevitably you wind up with an "HM whore" or two.
They're a pain, what with being magically unlearn-able unless you bring it to some mind-warping man in some city.
From a gameplay standpoint, it's necessary to prevent you from trapping yourself. Suppose you lack Fly, you fight somebody on an island, and forget Surf? Or you forget Strength in the middle of a cave?
Hm, you know, if they were to make HMs usable by the trainer directly, I think Teleport would have to replace Fly. It'd be pretty weird for a disc to enable kids to soar through the air.
