rtorrent is fast and small. from the beginning it was designed for server use and high speeds. it never had a gui nor any other bells and whistles the typical desktop user is looking for. it's a torrent client and that's exactly what it does.
it further can be remote controlled via xmlrpc/scgi which makes it very open for any kind of third-party or custom additions. therefore it can be included in automated setups or any other kind of use that requires some automation.
in both regards it's far ahead of the competition hence still the #1 choice for any sort of more advanced setup.