I have a Axiom Streamliner rack on the back of my Spesh Allez. I leave it on because sometimes I use my panniers but most of the time I have a messenger bag I carry everything in. RichardGlover pretty much summed it up: Sometimes you just need to carry a CC card, your ID and a Clif bar. Other times, you're packed to the gills. Having a rack keeps that flexability that most of us commuters crave (no, need!).

Also, what kind of question is that? What commuter bag can you strap directly to the frame? If we're being technical here, only bento bags and and frame wedges actually strap to the frame and they never hold much else than a spare tube and a small multi-tool (if that). If people are commuting with bags that small then I must be missing something.