I think you've brought up a good point here that each stone has to be treated individually. You're right that granite is porous and not all limestone is very porous. Following on from that though I'd say that some polished stones benefit from being sealed, some don't. No blanket policy.
As for polishing exposing/closing holes - I know that the polishing process exposes holes on a polished porcelain. When you say that it actually closes holes is this on things such as polished marble and other such natural stones?