That's most RAR users' reason for preferring RAR: Part of the scene. A standard. A sign of doing things like the black-arts-pros do it.
None of these are valid reasons. There was an argument that RAR was faster or that RAR achieved smaller sizes, and this holds true versus ZIP files. But the same people will insist on splitting RAR archives, and creating non-MD5 sums and generating an extra PAR parity file when in the end, they're going to use a Torrent and not Usenet to move the files. In torrents there's no reason for any of that. In fact there's a strong reason not to compress, so the file can be used while being seeded.
But as you can see from here already, the value of having a good version or implementation of the compressor and decompressor can not be understated, and WinRAR just fails that test.
7-Zip takes that cake, and generally does better for size and speed. BZip2 really should be in the running, but lots of people don't have a good GUI implementation. The command-line is great of course, but right clicking like 7-Zip, or drag-and-drop like StuffIt is just so much easier.
Here's someone's 2002 measurements that seem to put RAR ahead. But multi-threading and memory use are allowing for changes in this area that seem to leave RAR behind.
P.S. The worst example of compression used badly is when I see image, video and audio files that are already compressed with a lossy compression like JPEG, DivX, or MP3 further "compressed" with any lossless format. I'm sorry but it should be obvious that in most cases you're not reducing the file to less than 95% of the original size, and in that case you're just wasting everyone's time and efforts.