Last year I was drafted a couple times, very similar to wunderkind's scenario. The first time, I had no idea he was there, and didn't see him until I shoulder checked before moving left to make a left turn. Scared the crap outta me to see him there, actually. The second time, it was a guy who I had commuter raced a couple times already, and I knew he was there, but I guess he just wanted to draft instead of properly challenge me. He eventually passed me shortly before my turnoff, and even thanked me for the draft.
I try not to draft anyone myself on my commutes, though. I don't know where anyone else is going or if I can trust them to not make sudden moves, so if I find myself behind someone that I'm not in a position to pass, I'll hang back a few meters.