I think this debate will go on for perpetuity. I use clipless, cranked down to the max on the road with the fixed-gear commuter. It's just easier when running winter shoes, etc. But I use clips and straps when racing, for a number of reasons, most of which have been laid out by carleton, but to add a few:

--Shoe upper flex
--breaking cleats under effort
--pulling cleat out of bottom of shoe

Speaking as one who has those last two, nothing good comes of that. But for most single-speed riding, clipless is just fine, though my vast preference is for a road-type pedal such as the Shimano Dura-Ace.