Seriously. I must be going to hell because I do this, I even ride my nice race bike sometimes (the horror! :-) ) (if I've gone for a long morning ride and decide to swing by home, get my backpack and ride the 8 or so easy miles to work, why should I change clothes or change bikes?

On a more serious note, I actually ride my road bike shorts because they are much more comfortable than the mountain bike shorts I own. Also, OP, you might want to try a narrower saddle. I had problems with my inner thighs chafing but its because I have big thighs. Once I switched to a narrower saddle (Fizik Arione), all was well.