The USB printers can be cheaper since the connectors and cables are that much less. The USB transfer rate can be higher than parallel and can result in less CPU load since most OS's 'poll' the printer port. USB is interrupt driven so you usually see less spikes on the CPU chart.
Let's bottomline this. Why not just connect it and use it?
Canon's i560 inkjet printer comes with both a parallel port and USB interface. Will improved data throughput of USB really improve printer output speed or quality? Will the use of the USB rather than the parallel port put a bigger or smaller drain on computer processing speed when printing? Do printers benefit from USB 2.0? Are there other specific benefits to using the USB connector on an inkjet printer, rather than the parallel port interface?