[THEOSU]

I'm going to bring a bit of a different perspective...

I've had reasonably serious talks with a player who I find decent (but who thinks he's better than I do) who uses this mini raise to take control of the hand. His theory is that he will miniraise in a spot where he thinks his opponent has nothing, then lead out the next street. He finds this method extremely successful in winning a number of pots. I've seen a few pros use this sort of play in tournaments, too, although not quite as inexpertly as the player of whom I speak.

That may be the sort of thing that's going on here. Villain may be thinking he can get a little more in the pot, then bet any "safe" turn card (read: non-ace) and induce a fold. A call here allows Villain to continue his shenanigans, and therefore allows us to extract chips.

As others have said, raising here will allow villain to get away from a number of his weaker hands, while jamming with hands that we can only hope we're flipping against. In addition, the range of hands I'd simply call villain's checkraise is awfully big, and I don't really want to tip the strength of my hand with a raise here. The reason we raised with this hand in the first place is to play a well disguised hand in position. Why stop that now?