Lies.
I've only seen two "good workers" become part-time supes. The others were slackers and
-ups at best on the preload, and a couple of them were just already supes when I started and have stuck with it....
The ones that were supes before I started? They're actually the best ones to work under in my center....
But no, usually, in my experience, they ask people that just finished their probation. It seems to have nothing to do with their ability, and everything to do with the fact that they're brand new, seem to be able to at least hang on with the workload, and haven't been biased one way or another yet (and the union hasn't had a chance to get to them yet, either).