This is actually a variation of the "because the pros do" argument...
I can speak only for myself, but I've done the same 25-mile RT commute thousands of times since 1979, on I don't know how many bikes--a recumbent, my singlespeed, at least two mountain bikes and four or five conventional bikes including my Atlantis, Rambouillet, old Trek and a Specialized Allez. Tires have ranged from 23s to 41s on the roadies, and from knobbies to 100psi slicks on the mountain bikes. Pedals include platforms alone, with toe clips and Power Grips, BMX flatties and two clipless systems.
At least for me, the pedals make no difference at all. Tires count most, and how I'm feeling that day is probably second. There's NO correlation between my fast or slow days and the pedals i'm using. I'm not saying they don't work for some people, at least psychologically, but over roughly 3000 identical rides, they haven't made a difference for me.