Rdio can be run on a computer without a program. Spotify can't. Perfect for people who can't install things on their work computer.
I actually can't install anything on our office computers, but for SOME reason Spotify is pretty much the only thing that has actually been allowed to be installed without needing to be an administrator. I beat the system!
From all the responses I've been reading it doesn't appear that there is really anything that rdio can offer me that Spotify can't (at least to the point where the benefit was worthwhile enough for me to switch) - but I can see why Rdio would be more convenient for others.