What do you mean by necessary? If you never have need of it , then it is not necessary. If you do, then it is necessary.I went to download the SL 10.6.2 update and I saw Stuffit Expander as a popular freeware download.
What can it do that the normal install of Leopard/Snow Leopard can't?
Apple had already incorporated .zip capability into the MacOS X Finder. It stopped bundling Stuffit Expander with MacOS X.
Those are the main reasons why I don't use Stuffit Expander anymore. I don't run into many files that can only be opened by Stuffit Expander and I hardly ever compress files so I don't really need Stuffit Expander for my compressing needs either.Nowadays, OS X can handle most compessed file formats. Most things are .zip or just in a .dmg file, which OS X can handle.
I don't know much about unRarX, but looks like it hasn't been updated in 3 years, so I think your best bet is The Unarchiver. I used to use StuffIt Expander for months, but recently started finding out the hard way that it just won't unRAR password protected files. So I had to ditch it. With Unarchiver, however, I have yet to encounter a problem.
You can download it here. It's free: http://www.macupdate.com/info.php/id/22774/the-unarchiver