45 to 75GB IDE hard drives can be had relatively cheaply these days, particularly when you consider the cost of large SCSI drives. Moreover, you'll also find removable drives, like the Jaz, Zip and Orb with FireWire interfaces. More FireWire scanners are appearing, although at the high-end right now. But that'll change fairly rapidly. I've even come across some FireWire inkjets lately - again, at the higher end of the market.
Cost is one reason.
Although there's a bit of capital investment involved in acquiring a FireWire setup, it's a lot more future-proof. Not much use today. The difference is that your FireWire enclosure will have a long and useful life. Ditto for when your trusty SCSI CD burner grinds to a noisy and smoky death. Mind you, by then, we'll probably all be using DVD-R for recording gigabytes of data.