So far we know that AP can be diminished via environmental hazards, offensive items and throws while Sly is in the only character so far to have an ability to outright stealing AP with his invisible backstab/pickpocket.
While this factor could affect the decision, presumably items and hazards would be turned off in tournament-play, leaving only one means of AP-bleeding, which may become very unlikely to land for certain characters. If we go in the direction of AP-drain as a viable tactic for more characters on the roster, we might see characters based around it with much lower combo-potential (dividing the cast into combo-slingers vs. leeches). Alternatively, if we let Sly keep it to himself, he could theoretically become extremely high-tiered (though since he's slightly visible, good players will eventually learn to spot him).
So do you want this technique to belong uniquely to Sly or do you want to see it much more often as the reveals arrive? Do you lean towards a game based more on combo-stringing or resource-denial?
I like to make PSASBR move-sets. Here's a few:
Never say I don't contribute: I'm The Idea Man.
Well, Sly's move cancels out because it's much riskier to attack with him, as he can't defend. It's pretty fair from what we have seen, honestly.
No block = AP steal ability
He has to get really close to get the AP steal move in on people and without the block, that's not something you just do