I think it isn't zipped only for make it simple for users to know how to make a skin.
Maybe the loading time is shorter with not-zipped skins, but the difference isn't noticeable and once the skin is fully loaded, the skin file is not inspected anymore.
I don't think there is a difference if they have rgn.dat file with the file. I've just recently been told that the skin loads faster if it has a rgn.dat file with it.
Email form bst:
One thing, include file rgn.dat in zip(bsz) file.
In this file window region data is saved and skin is loaded much faster if this file is present.
This file is auto generated the first time skin is loaded if it doesn't exists.
But if skins are in bsz format, they are unzipped to temporary folder and deleted at player exit, so it's recreated at every start.
Also, everytime you change main picture for skin, press CTRL+R to refresh skin (and recreate rgn.dat) or delete file rgn.dat, otherwise new changes won't be visible.
Had to re-do some of my skins.
|bsz, fast, unzipped|