Why should you scrub/clean cast iron cookware with salt?
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.