AO is one of the quickest ways to get "that GI" look, which is why it is used more and more in movie productions where a full GI solution is still to expensive. Also, AO usually is the easiest to control, as it only attenuates your existing lights instead of adding extra indirect bounces. With full GI, you can't really tell what your lights will looks like until you render, where GI even can be rendered out in a separate pass and reused again and again and composited over non-AO passes in Photoshop (Poser 6 comes with a "render passes" script that can help you there). IBL is useful for integrating 3d objects with existing 2d footage, but it's not very good when you have exact ideas ("I want a soft blue fill from below and a bright white highlight from the upper left").