this just occurred to me.  is this assuming you're already using very accurate directional lights?  that is, if, with no IBL, if your image produced glowing areas like nostrils, parts of the ear, etc.,  wouldn't you still need AO to affect the work of directional lights?  i ask because i'm sure i'm not the only one who can't get rid of horrendous grain in raytraced shadows, and i know for a fact that most people don't put up with renders that take as long as mine.   i'd bet that most people still use depth mapped shadows, and those can be hard to get to work well and on a detailed level. 

but maybe i'm wrong and people work with more accurate lights.