i also always wear one moutain biking or when riding my trials bike. there are some trails around here where i wear a full-face. i had a spill taking a nasty drop on my hardtail once. if i hadn't been wearing a full-face, i'm positive my dentist would've gotten that new m5 he'd been considering. my full-face gives me a lot more confidence. besides, what other helmet could also trap a bee inside it? now that is fun and a great reason to wreck.
when i go out on road rides that involve intense climbs and fast decents, i wear one. more mellow road rides, i do not. also, on the track i wear a helmet. you have to, but i don't think i would choose otherwise.
around town and commuting i don't wear one, but naz's poster always makes me reconsider that.