As a Meta-Knight main, I can say gliding is REALLY useful. His glide attack is one of his better knockback and % dealing moves, especially since you can UpB then glide attack your foe. Glide attacking is soo useful. Think of gliding and glide attacking as an extension of getting on the stage, except when you glide attack, you don't have the lag like a ledge attack. And you can fly over your opponents. And if you want, you can go under some stages, which is a basic mindgame.
Charizard should use gliding solely for recovering, he is too slow to use it as part of his attack strategy, and his glide attack simply doesn't work.
Pit, well he isn't as good at gliding as Meta-Knight, but I guess you could use it offensively, though from what I've seen his glide attack is ok at best. I don't play Pit if I can avoid it, so I don't really know how to play him. His recovery is already insane, so he doesn't need gliding, but you could use gliding to mix up your recovery a little.
Meta-Knight, again, his glide attack is useful, and it's easy to start gliding, and yeah, Meta-Knight is the fastest glider.
And besides, once you have mastered gliding, you can have fun with it and do tricks. Then your friends, if they are a glide-capable character will try to copy you, and fall to their deaths, since they don't main gliders. Gliding is useful, that's my opinion! (I also think it is fun)