Interesting idea, Dan. Do you propose that we should make it so that every pokemon gets up to three abilities? Or do you think that we should just let everyone change on the fly?
We've thought about incorporating something like this down the line once the Move Tutor shop took off, but we haven't really been able to iron out the details.
As for Pure Power Slaking -- sure, base stats aren't to be taken into account, but the overall size of a creature definitely does. Slaking's traits are that he's ridiculously strong (which should be reflected in battles), but he can't really be arsed to do what he's told. Giving him an ability that neutralizes his laziness without handicapping (instead, rather, increasing) his ridiculous strength would make him broken, I'd say...
What we should consider doing is keeping the worthless abilities, but make the pokemon who have them capable of giving them to their opponent (via Entrainment, which is now a move tutor, or Skill Swap) in order to level the playing field.