I routinely wear street clothes (and a backpack) while riding my $3.5k carbon race bike. I like to ride it, so when I can I commute on it. No point in full kit to go 3 miles to work...

I do tend to wear my bike shoes though, only because the contact area on my pedals is puny (mountain bike pedals).

But yes, I have seen bikes that I thought just HAD to be stolen. Like a nearly new, but obviously abused Specialized, must have been a 3-4k bike, with very old school cage pedals that were beaten to heck. The rider looked like he hadn't showered in a week and his clothes and shoes were in dire need of replacement...