AFAIK, the original ATX spec called for a fan blowing air onto the CPU to help it cool. Eventually people realized that blowing hotter air on top of the CPU wasn't that great of an idea (especially as CPUs started needing better cooling and systems started drawing more power), so it got switched back to the old AT-like style of having the PSU as an exaust. I believe having the exaust at the bottom was mostly introduced back to cater for cases where the PSU is mounted at the bottom, so they draw cooler air from the outside and take no part in the case cooling solution.
EDIT: Also, I believe having an exaust at the bottom of a PSU would actually hurt CPU cooling in an old ATX case where the PSU is mounted facing the CPU. In the original specs the PSU would blow air onto the CPU cooler, therefore helping it. An exaust fan would create a low pressure zone, reducing the amount of air the CPU cooler would be able to blow. On newer-style ATX cases with the PSU on top this would likely not be a factor, as the exaust would also help remove hot air coming from the sides of the heatsink.