The problem is that in a lot of places, bike racks are the only place to park. On UT campus here, a bike that isn't parked in a bike rack (for example locked to a rail or a tree) gets swooped on, lock cut off, and impounded by their PD in minutes, so there isn't much choice in the matter -- the bike rack or walking.
Another problem with the single lock method is the fact that without locking skewers and a bike parked in a very theft-prone area (Austin where I live used to be on Kryptonite's top 10 list for a number of years), a thief will look at a bike that has a rear wheel locked... pop the QR on the front wheel and be off with that as a consolation prize. A similar thing happens with seatposts. With locking skewers, one Sold Secure gold-rated U-lock is good, but without, two locks are a must, in order to come back to a bike that has both wheels still present.
In areas where bike theft isn't such an industry, a single lock is adequate, perhaps with a cable to go through the wheel and seatpost.
Last edited by mlts22; 10-19-06 at 03:49 PM.