The desktop client is what I am talking about, it will try and use as much upload bandwidth as you have if you put popular tracks in offline playlists.

AFAIK, the Spotify desktop app throttles the amount of upload so that it shouldn't disturb the user (or at least, it used to do that). If it used up all my upload bandwidth, I would consider that a bug and file a report.

I did, they dicked me around and suggested to disable offline playlists or reduce my cache size to fix it.