Why do we have to do a dirt comp ?

[Morbid Angel]

that's why i never understood the mental ray tutorials that come with max, that teach you to combine final gather with ambient occlusion on the walls. purposefully darkening the corners, where actually the final gather result without AO was far more realistic. As vlado mentions in this thread white walls wouldn't be darkening in the corners which is what the AO does.

I guess it's just teaching you techniques though...........but i think it could be done in a much better way

Keep in mind, that those tutorials were probably written by people who did not fully understand physical lighting so they just assumed something without thinking them through.
It used to be this way:
In the older days, when stuff was lit by spot lights, directional lights, point lights, using non soft shadows, or perhaps fake depth map shadows, the details on the geometry were very much lost. Keep in mind also that amb occlusion comes from render man, from ages ago. This method was developed to solve this issue of lost detail.
I dont think it was ever intended to do more then that.
Nowadays when we have area lights, soft shadows, gi, etc for the most part it takes care of the (ao effect) and often I hear from people who switched to vray, that they no longer need the ao, since vray provides the much needed result.
Though, percy would say that it does add a nice detail and I would agree. Naturally, we tend to use biased methods, like irradience and lightcache, which overlook detailed geometry as well. If we were to always render with monte carlo, or progressive paths, there wouldnt be a need for ao.
Ao in interiors is especially tricky, it can be applied to certain objects, but most definitely not to the whole scene.

[Morbid Angel]

I will also add, that AO is a great tool for texturing. Baking ao, and creating worn effects based on that works very well. I couldn't live without it