>and I like my bike to be balanced and as light as possible.

Funny, for touring or general getting around, I agree with you, but for commuting I find bike balance doesn't matter as I'm going in straight lines on a very predetermined course. There's no real manoeuvring where it would be a problem.

Though, since I have a shower at work now, I'm tempted to try backpacking it once just as an experiment.

The other option no one has mentioned is a rack top bag. Slightly improves handling and aerodynamics.

I've also commuted a couple of times with just a handlebar bag.

Steve