Well, this is similar to Apple's requirement that Tiger needs USB to run. If you want the HD out of an iBook Clamshell, install Tiger on it, and put it back in it runs it just fine with enough RAM.
I agree, it should be a case of checking against the minimum requirements for processor, RAM, HD space etc and warning you that installation could cause instability. In the case of checking for USB though, that was more along the lines of some machines without USB wouldn't support the OS, (or should I say the OS wouldn't support them), and to make it easier, they lumped all machines not having this as one. As far as I know, all PCs with the minimum RAM will run Aero, but Microsoft don't let you select it with less than 1GB. If this is the case, a warning and proceed/cancel would suffice in my mind. Most aren't stupid, but many aren't too confident when it comes to computers, so they'd lick cancel.
Just to correct a little though, Panther required USB, Tiger required FireWire.