[gliese1337]

I think you presume too much about the motives and planning behind the majority of car thefts.

A professional car thief who steals cars in order to resell them may very well work that way. But I suspect those people are a small minority in comparison to the opportunists who probably don't have appropriate spare plates on hand, or the desperate criminals trying to pick up a getaway car, or the joyriders, etc.