I agree with most posters here in that if you don't trust the lock not to get stolen when it's there by itself, why would you trust it with hundreds (or thousands) of dollars worth of bike?

As far as using locks to reserve spots, i'd say it's rude to do, but i'm sure everyone does it anyway. Just ignore the locks when locking up. It's not like they can't unlock and move their locks when they arrive.

I carry my mini U-lock in the bottom of my panniers at all times, just in case I want to stop in at the store on the way home. But, if you're not worried about stopping anywhere other than work & home, leave it there.