I'd leave ANY of my bikes in a rack if I needed to - they can be replaced. I normally don't, however, because there are few places I go that have racks. The more expensive the bike, the less time I'd leave it in the rack. Also, some of my bikes are more attractive to miscreants than others. My full-campy Outback would be an attractive target, but my home-converted fixed-gear (with my name all over the tubes) probably wouldn't be.

The ONLY bike I've ever had stolen was a Sears single-speed coaster that I parked (sans lock) behind the A&P grocery where I worked when I was about 17. When I went home that night after work, I forgot that I'd parked the bike in back, and the next afternoon, it was gone...

Other than that event, bikes don't seem to be targeted much by thieves around here. Guess we're just lucky!