Well... I have a case with 6 fans and I just want more airflow lol. It
helps, especially when its a 120mm fan lol.
If you really need 500W PSU (which is pretty hard to "need"
even with most full tower setups), it's then understandable
to have 6 fans especially when they're optimized for lower
RPM, quieter operation. Main issue then is which parts are
most susceptible to the heat.
Suppose you have a very well stocked/modern system that
manages to use 350W at full load. At 70% PSU efficiency
that's 150W of heat created in the power supply... it may
not be ideal to have the power supply drawing in air
pre-heated from the CPU. If it's that difficult to keep CPU
cool enough there may be other alternatives like room AC,
water cooling, a better heatsink, or a duct to either pull
in cooler air or more directly remove heated air out the
back of the case.
A second advantage to PSU not having a fan on the bottom is
that there's more room inside. Past about 400W it gets
pretty cramped in any PSU actually capable of sustaining
it's rated wattage, adding a 2nd fan can be good for
marketability but bad overall for the power supply design.
Antec is one of the better models you could choose though if
you did want one with a fan on the bottom, but if your case
is roomy enough there may be another alternative too, taking
a PSU without the bottom fan then cutting out a fan hole,
mounting a fan on the outside wall of the power supply
casing. This may allow a thicker, more efficient fan and if
it'll fit next to the CPU heatsink, it'll be nearer to it so
a larger percentage of 'sink exhaust is drawn away than if
the fan were recessed inside and further away. However for
the benefit of the power suppply this may mean the PSU needs
higher flow rate to remain at same temp... the heat has to
go somewhere.