Why should you scrub/clean cast iron cookware with salt? I've seen this a number of times (for example here, or here at cooking.SE), and I do it at home, but I'd like to know for what reason salt is good for cleaning (cast) iron cookware.

I know it's a stop-gap measure when a non-stick pan looses it's coating (I can't find the reference).
The purpose for cleaning with salt is that it is an abrasive, and helps remove anything sticking to the pan, without being harsh enough to remove the seasoning.  

Its not a stop-gap measure for poor seasoning; it is intended to preserve the seasoning.  The main issue is avoiding soaps and detergents which would tend to remove the seasoning that makes cast iron fairly non-stick.