The same problem applies as to streaming from other services. If you search you'll find the threads relating to it.
They basically boil down to this: Audiosurf needs the entire track before it can analyse it accurately, this means a long wait whilst Spotify grabs the first track and again for every 'skip' performed.
It's not a total deal breaker, and there are a lot of other minor problems with it as well, but that's the main one. Especially if Spotify will only let the app consume data at the song speed, as you effectively have to wait for the entire song length before you can play it (not ideal).
This also assumes that Spotify won't object to you keeping a local copy of the track for as long as it takes to surf (they may, or more accurately, the media companies pulling the strings probably will).