I'd be interested in some feedback on this as well. I just wear 501s too
For me I find its very convenient to just go and jump on the bike and ride it in whatever you have on if its local short rides ....maybe its just my inherent laziness but the thought of having to change all my clothes and shoes every time I go out for a ride would put me off using the bike.
That said ,..the one time some idiot knocks me off my bike I'm sure that as I slide along the road on my arse the only thought that is likely to be going through my head is 'I wish I was wearing some of those kevlar pants '