metal drive enclosures that are quite efficient at ducting heat away from the drive unit.
While I agree that some cases are better than others, it is indisputable that a drive will be warmer in a sealed box than in an enclosure that forces air across it.
If heat is the enemy of all electronic devices, which it most definitely is, it makes sense to run them as cool as reasonably possible.
I'm using eight 300 gig drives and had one of them fail recently after less than a year of use. I'm blowing some air across mine.