All of the speed stuff aside,
I have seen many people attaching multiple drives to the IDE bus and switching the power to the drives to control what's "on" the bus. There is one serious flaw with this which may not be evident to people. The connected drives which are not powered are still physically connected to the IDE bus. These drives still represent an electrical load. The drivers in the active devices need to work harder to overcome this additional load. Ultimately this will reduce the service life of the drives and controller. Whether or not something else dies first. . .
Granted that is is working for several people, I'm just waiting for it to kill someone's box that was marginal anyway. The IDE specification defines 3 devices total for a reason.
-nB