I hate this too! For me, tho, the worst part is that they're not even consistent. I never know if the computer will pick up whatever I'm playing in the phone: sometimes it does, sometimes it doesn't. I don't want to check if my phone is connected to the same wifi for it to work.

Back when I used Rdio, the behavior was to keep all devices/tabs synced. It was consistent. It worked. When I wanted to keep diferent sessions I used playlists. I could work around it.

But with Spotify it's a mess. It doesn't sync correctly when I want, but only when I don't want.