Originally Posted by tjspiel

This is not a recommendation but here's few reasons why people use them:

1. They've already got one.
2. They ride hard and get sweaty anyway.
3. They don't like the way their bike handles with the extra weight/aerodynamic drag of loaded panniers.
4. Backpacks are nicer for carrying their stuff once they get off the bike
5. Backpacks work on any bike, rack or not.
6. Having extra bags/panniers makes the bike less manageable in crowded situations: trains, elevators, etc.

It's probably worth pointing out that some backpacks do a reasonable job of getting air to flow over your back so that sweat isn't as much of a problem.

Even considering the above, any solution that allows you to carry your stuff on the bike has obvious advantages that for many people are well worth the trade off. I think that's the key point to remember. There's not a single *right* way of doing things and its good that there are so many options.