For starters, it just works. A good friend has a Motorola Droid X. Android is a useless piece of shit -- his alarm clock keeps crashing with NPEs. This was one of the premier android phones when he bought it, and he left it completely stock. I, along with many friends of my generation, use our phones as our alarm clock and watch, so this is obviously pretty useless.
Second, my three year old motorola krzr is robust. It's been face down in my pants pocket while sliding down 500 feet of ice when I wiped out on some really icy moguls last year. That shattered the front. It's survived prolly 100+ impacts in my coat while snowboarding, though none as brutal as the above. The paint is peeling off the entire back. It's dented, prolly from a different fall snowboarding. It fell out of my coat pocket and hung out in the snow for 8 hours next to our car last year whence I fortunately saw it before driving off. Through all of it, it still makes / receives calls and makes / receives texts. I'm pretty sure I'd kill a smartphone inside 5 snowboarding days, tops. I still get 30 hours of standby battery life, though I'll probably have to replace the battery soon. My friends w/ iphones and the like seem to rush between charges, and last weekend at the ski house, there was a rush to find the one person who brought an iphone charger. I didn't charge all weekend.
tl;dr -- dumb phones are robust, have great battery life, and generally work well at their limited functionality