USB isn't worth it on a machine that already has SCSI. SCSI is 5 times faster than USB for practically all purposes.
If you want to add a scanner, SCSI scanners do cost 2 to three times as much, but they are worth it. Epson has been good enough to
make its Stylus 740, and 900 printers both serial and USB compatible. So if you were looking to get a printer, I'd get either of those because that way you can print now, and later when you get the newer technology. SCSI is still the only technology you can get a cheaper CD-R drive. CD-RW is the only recordable CD technology available for USB, and is less reliable.
The only reason you might want USB is for gamepads which are practically all USB now. But for that purpose you could buy your own Playstation or Nintendo and attach it to your TV for $100.