Well, if you are into bicycle themed fashion, then sure, go ahead and wear you spd's all the time.

As for me, when I'm going to the class/store/movies/girlfriend's place/work/etc (I use my bike for everything), I don't want to be wearing biking specific shoes. In my opinion, they don't look too hot.

When I need to ride for real, as in non-just around town biking, I switch pedals (ie, long weekend rides, training).

Using a good pair of toe clips and some stiff leather straps should be sufficient for riding around the city, even enough for moderate skidding and whatnot (but you should have a brake for that either way). For the kind of riding I do, lots of stopping, going into places, etc, I don't need/want spd's. I could see a messenger using cleats all day, since they are mostly on their bike, but not me (and I would venture to say, most people).