I don't know if they are theft deterrents, but removable pedals are a hassle to use when commuting esp. on a train. Even if you just remove one pedal, you still have to put it somewhere and the spindle gets dirty. The less removable parts to keep track of, the better.
More importantly, when you're just cruising around town with t-shirt, shorts, and sandals, it's really uncool and uncomfortable walking around with a pedal in your pocket once you've reached your destination.
