Gigabyte has been a big player in high performance with passive cooling. I've used such designs in their 7600GT and 7600GT. The big advantage is that there is 0DB noise coming from the card - okay, I am assuming that.
With a big case cooler fan, it helps to suck out the heat generated by the radiators.
As shown on this ATI5770, the 2nd slot is used as an exhaust vent. This CUSTOM design use used by Gigabyte and looks very much like my 7600GT from years ago... and it was HUGE back then.
For heat issues, I switch to BIG-FAN coolers like HIS graphics cards which come with ICE coolers, which I had before my 7600GT. My 4670 is ICE and silent.
Paid $180 for my 7600GT - Passive and $80 for my 4670