pcar 930- With heavily oxidized s/s, you often have to remove a lot of "dead" paint to get down to the "good" paint you really want to work on (and yes, for you experts, I know there are a lot of related issues here). A quick pre-compounding, so to speak, before starting the "regular" process (clay, polish, etc.) can get you down to what it's really all about quicker. We're talking about some pretty extreme situations here, and, FWIW, *I* have always clayed FIRST, even on really crappy s/s paint.
With basecoat/clear, if you have THAT much "dead" paint (which would be clearcoat) to remove, you're probably looking at a paintjob anyhow. And clearcoats just behave differently. You really shouldn't remove more than a TINY bit of clearcoat, what is it..maybe, .3 of a mil..that's not much (consider the decimal position). Which is why I sometimes worry about the long-term effects of all the polishing some Autopians do.
Jesstzn- Yeah, they have lots of oils and some fillers. Especially the Metallic, which actually contains WAX. This hasn't caused any problems for ME, but it IS a reason why they might not be right for everyone. As SamIam mentioned, the 1Z PRO line is supposed to be free of such things, but I haven't tried those products yet.