I think the value in a helmet if more for the accidents that happen so quickly you don't have time to react. If you clip a car, sign, whatever, and have time to realize you are going down you drop the bike and roll. There is always the chance you won't see it coming: door prizes, ice on the street, pedestrians stepping out from between cars (of course then you make sure you land on them, nice and comfy), wet storm drain covers.
I was riding after it rained, turned a corner quickly and my front wheel hit a wet storm drain cover. Zip, no more traction and down I went. I felt my nose brush the tarmac then my helmet hit and bounced my face away from the street. No helmet would have meant me leaving my nose and face streaked across the tarmac. It's not like I had a modelling career, but holy f*ck am I happy I was wearing a helmet.
On a less serious note, you spend a lot of time as a bike courier rushing around madly and more than once I've rushed up to doors, pushed the bar and found them locked. I'd bounce my helmet off the door (not my head!), curse at whomever locked it, and move over to the other door.
Oh yeah, if you mountain bike, wear a helmet (assuming you are doing serious mountain biking, not just toodling down fire roads). I've messed up a descent and scraped my helmet across a rock face. No helmet would have been very, very ugly.
Of course, to each their own. I don't feel modern helmets are so restrictive that they aren't worth wearing.