I've usually used a heavy steel bike, straight gauge tubing...mostly because of the lock up thing. We use those heavy $100+ chains here; If you ever watch people locking up in crowded areas, you'll note that one guy will carefully pass his chain with the cloth sleeve through his spokes, being gentle with his paint job as he tenderly locks his baby to a sign...then, ten minutes after he's gone into the office, some other yutz comes along, and in the process of locking up to the opposite side of the same sign, throws his bike against the first one, and carelessly bangs his chain against both frames as he locks up. That would explain some of those mystery scratches...

I've seen people lean on parked bikes to make a cell phone call, etc. I am sure an AL or titanium bike would be more rust proof but I like knowing my frame tubes can take a little knock and not crumble.