The above answers are absoloodle correct. Although I never heard or thought about the cooling effect when the A/B is open. But as far as I know, no pilot ever leaves the A/B open because he or she thinks of cooling the APU bay/packs etc. It's just the wind and the fact that the hydraulics are de-powered. The normal lever position of the A/B & Lift spoilers is left in the "IN" position (on ground). Otherwise you'd have the lift spoilers deployed too (provided the hydraulics are powered). So I don't think the cooling thing is really a reason. But hey, who knows? It could actually be a good idea. Are there any mechanics out there who'd recommend this?