I'm much newer to cycling (April 2008), but I commute much as you do (except I live closer and ride the whole way). There is no need for specialized cycling clothes on casual short(ish) rides.

On the other hand, I generally use special clothes for cold weather, hot weather, long rides, races, etc. I almost always use platform pedals, but I may use clipless for an upcoming race (Gravel Grovel).