A lot of people are mentioning dual SPD/platforms, how they almost never end up using the platforms.
Right now I have SPD on my commuter, and platforms on my mtn bike. Frankly, I like my commuter better (Surly CrossCheck), so if I need to jump on for a quick ride to the park or something, I'm more likely to just pedal my SPDs in tennis shoes (not that hard). And if I go on a longer ride with the MTB, I swap the pedals so I can ride clipped-in.
Pretty soon I plan to get some dual pedals for the MTB, so I won't have to swap pedals anymore, but I would never want them on my main bike. It would be a major pain to have a 50% chance of failed clip-in every time.
But it is personal preference. Surely it's a balance involving how often one is likely to ride in street shoes, and how annoyed one gets at clip-in fail.