Very many new computers don't have a free IRQ available to install a SCSI board, and many don't have an empty ISA slot either. So most scanner companies have largely shifted to parallel in self defense for the PC models, to minimize support problems. It is a real pity our PC architecture is so limited. Macs support SCSI out of the box, except the new iMacs don't, they have USB now. Laptops typically have no choice but parallel either.
Parallel scanners do have the advantage of being more easily used on more than one computer.
However, parallel ports are not designed to support multiple devices. Many of you already have a printer and zip drive or tape drive already on your one parallel port, and if your intention is to add yet another device there, then I worry for you. Reason says all those different drivers watching the same one port can only cause trouble. But if it has to fight other drivers to access the port, then I would expect some problems. The problem is typically the printer, and some printers are better behaved than others.