Rar archives are generally less effectively “packaged” than 7-zip archives. For some types of files, 7-zip is a lot more effective than what WinRar or WinZip can do.
To give you an example: I work a lot with database dump files and log files. By fiddling with the archive settings I get a lot of those 4 GByte dump files back to around 100 MByte. I’m happy if WinZip gets the same file back up to 700 MByte, with WinRar I’m happy if that is 500 MByte.
I often have to transfer these kinds of files over the internet. The extra time it takes to get that 7-zip file that small is not that much more than with the other archivers. The time savings during transport that these much smaller files give me is considerably higher. Smaller files are also generally less prone to corruption in transit.
So since I started using 7-zip, I’m no longer of any use to those other archivers. It does exactly what it’s supposed to do, doesn’t cost you a button, you can ‘play around’ quite a bit with the settings of the 7-zip archive format if you’re interested (or not at all, fine too) and you can download themes for it , which allows you to customize the icons and GUI.
The reason you can’t create .rar files with 7-zip is because RarLabs didn’t open up that archive format. They should of course know that themselves, it is their product. However, I am much happier with 7-zip than with WinRar.