Actually, for Mozilla Firefox users, if we click on the .8xp file, instead of downloading it, it automatically views the file (incorrectly) in the browser. If we put it in a .zip file, it downloads instead.
.zip is a universal format, and most of us (the world, not this community) can't read .rar files, and can read .zip files. And most of us don't want to download extra software to read it.
.zip is also readable by Windows 98 (later versions), Windows ME and Windows 2000, Windows XP and Windows Vista, without any extra software.
Does that solve your question?
Cheers ~ James Kanjo