Well before this turned into a bickering fight (stopped reading it properly about halfway through page 3 when people started getting silly) I thought Id reply...

Riding into work each day I dont wear a helmet.  I have no doubt its a good idea and Im kind of suprised people are saying its not.  My helmet fits well, doesnt hinder movement and theres no way it makes trees uproot themselves and attack me lol.  It helps in a small set of circumstances and I wouldnt try and claim it doesnt.

However, I personally find wearing it a pain in the ass for quiet street riding and zooming through parks (I pick quiet routes and tend to get off at busy intersections).  They make your head warmer than not wearing it (no matter how good the design), you pretty much look a bit of a prick and then you have to babysit it during the day or use up your bag space to hold it or lock it onto your bike.

I choose not to wear it as the benefits are small enough that the inconvenience of having it is too much for me.  No doubt stupid but its my personal choice.

On the other hand when Im chucking my bike around (mountain biking) I sure as hell use it because slipping on rocks, hitting trees or just smacking a landing too hard is all to common and hitting your head happens with when your pushing it or misjudge how slippery something will be when wet.  On roads I go out of my way not to get into situations where I could be knocked down but it will no doubt happen some day.  Until then I turn up to work not looking like a sweaty headed freak.