Another good reason is that your feet don't slip off the pedals when it's wet, when you're making a lot of effort going up a hill, etc. That was the original reason I switched, because I ride through the winter here, in the rain, and my feet would slip going up a particularly tough and traffic-ridden hill, and it scared the crap out of me.
"My predominant feeling is one of gratitude. I have loved and been loved;I have been given much and I have given something in return...Above all, I have been a sentient being, a thinking animal, on this beautiful planet, and that in itself has been an enormous privilege and an adventure." O. Sacks