Action Replay cannot read a rar file it can read zip. But ya your right it should be easier and it should be zipped instead of rar since Winrar handles zip also there is no excuse not to. But then again not all xboxers use that Action Replay product or AR Max they use the open source version from
http://www.xboxsaves.com/ since there are so many save managers out there its really hard to get mad. Just make sure for Action Replay your root is named UDATA/45410056/ (4 folders) then just zip the Udata up with all the contents and drag it in Action Replay will recognize it as a MVP save.