Minor detail: The Air app is still available and may work as a desktop solution on Linux.

http://www.rdio.com/#/apps/air/

(I personally love rdio and am completely confused by spotify.)

Matching iTunes collection only matches songs which are also available in Rdio's catalog. Everything else you'll still have to open up iTunes to play, and you can't even mix and match.

You sound surprised. How could that feature work otherwise?

Well in spotify your itunes music is imported into the spotify player.