I need to put a rack on this thing! When I put the weekend's worth of stuff, a laptop and today's clothes in the backpack, it became more than I want to handle. Maybe it makes sense for some people to carry stuff on their back. But if you're doing it because you think it's "good enough," I'd challenge you to try a rear rack and panniers. A backpack makes my back sore and it's a lot more sweaty than running a pannier set-up.
You can find this stuff cheap online and it makes commuting so much nicer. I have a buddy who runs these panniers and I use this rack. For $65 you can have a lot of cargo capacity and get everything off of your back. It may seem like a pricey leap, but once gas prices begin to get up there, it's negligible if it keeps you commuting more. When I think about building a bike, a rack is a non-negotiable. You won't regret it.
But for the sake of debate, anyone out there commuting regularly with a backpack who prefers it over a rack? I'd love to know why, if so.