I never really understood this. Certainly it's easier to bomb a crowded place by putting something in a locker, and leaving it there until it's convenient to detonate it (even if that's done via a timer that's set a day or two in the future)... but it's not much harder to just toss a bag with a bomb in it in a public trash can, or even just leave it somewhere vaguely-hidden. Sure, the likelihood of discovery is higher, but it still seems to me that it would work most of the time.