I don't think there's anything wrong with Use Magic Device as it is. Its relative unavailability (purely because it's a cross-class skill for almost everyone), coupled with the high DCs make it pretty much unusable until you're really high levels, at which point it won't be making a huge dent. Especially considering that at most levels, useful wands (such as Cure Critical and Fireball) cost so much that a character whose main focus isn't spellcasting (such as a Wizard or Sorcerer, who don't need the skill to use it) won't be using most of their money on something that their class doesn't even support. True, you could get a +10 modifier at low levels and have a pretty good chance of using a Wand of Fireball, but there's still a large chance you could have it blow up in your face. Plus, that's a ton of skill points you could have put into something more useful to your class. If you want a Fighter throwing around Fireballs at higher levels, go ahead, but he's gonna suck at pretty much all of his other skills.
The only real place I can see this skill being overused is on Staves- assuming a different type of spellcaster tries to use a Staff of a different caster and you allow him to use his own ability scores. (That's something I've been wondering about lately.) Otherwise, scrolls (which already have an amazingly high DC for anything useful) and Wands (which cap out at 4th level) already have set DCs, which won't be too useful at higher levels. I have a Cleric that has it as a class skill, and the only real times I've used it were to try and identify items by blindly activating them (DC 25). Even then it's only worked once or twice.
True, it COULD be abused with LOTS of careful planning and giving up of resources, but if someone does all that work to be able to use a wand of fireball, who cares?