I can think of all sorts of reasons I'd rather wear proper bike shorts (bibs, actually) than running/basketball/gym shorts:

1) catching the nose of the saddle
2) chafing on that big central seam
3) billowing material
4) and the fact that the pedaling motion will hike up the leg of the shorts and reveal far more skin than any pair of cycling shorts.

But wear what you want. It's all fine with me. I'm one of the douchier riders around and I'll accept you for who you are.