IDE devices on the same channel (sharing the same cable) cannot communicate to the PC at the same time. Only one device can 'talk' on the bus at a time. That's why if you ever do CD to CD copying with IDE drives that it's recomended to put them on seperate channels.
SCSI on the other hand can have all the devices communicating at the same time on the same cable.
IDE is limited to 2 devices per channel. SCSI goes up to 7 or 15 depending on which SCSI you're talking about. Maybe more now since I haven't looked into the SCSI-3 specs.
SCSI controllers offset the need for CPU cycles as the controller is doing more work controlling the devices. IDE devices require the CPU to do some work to controlling the devices.
Also... SCSI drives are usually of higher quality and considerably faster than their IDE counterparts.