Can someone please explain to me what the advantage is of 3-betting rather than calling down when you're OOP?

Assuming AA-99 and any Q aren't folding no matter what you do, then the only hands that you want out that might fold to the 3-bet and turn bet-out are AJ-A9 and maybe AK. The 3-bet also gives you the initiative when you don't want it if you're up against a big hand (i.e., it's going to get you raised on the turn or, worse yet, the river), plus, if he's bluffing or semi-bluffing then the 3-bet lets him get away from the hand cheaply.

So when you're OOP with a weakish and vulnerable hand facing aggression shouldn't the default be to call down and probably fold if an A or K hits?

Also, doesn't the fact that you're almost always calling down in these situations negate much (not all, but much) of the exploitability that might otherwise result from such a passive approach?