An interesting suggestion, but it would be tragic to downgrade from the Mac Pro case, which is considered the finest in the industry, to something like an Antec, Chieftec, Lian Li, Vantec, etc. I've got four tower systems I built and I got the best cases available and they simply are no comparison for the Mac Pro.

Plus, nothing would fit. There aren't Antec cases with the same ports (USB, FW800, FW400, Audio, etc.] built into the front and back panels. The Mac Pro's kilowatt power supply is unlikely to screw into a PC case. The motherboard is not, to the best fo my knowledge, a PC form factor like ATX. You'd lose ALL of the drive-sleds, not just one, since it's a proprietary system that relies on the Mac Pro's case design to work. Even if you got it all crammed in there, the entire algorithm for the cooling fan control would be hosed by the changes to the airflow characteristics.

If you need to swap drives that often, get a drive dock or an external FW800 case that supports slide-in drives.