Maybe I'm not understanding, but if you attach to a tmux session with the -d flag, it will detach the session from any other clients and resize it to the window you're using.
I'm very happy with screens implementation. It will stick to the size of the first client and you override it at will in any other client or if you want to update the size (maybe you changed the size of the window).
I don't want to detach my desktop just because I want to check current status on my phone. Maybe I have two sessions on two computers and I, temporarily for a few commands, want to go fullscreen. With tmux I have to detach the other client(s), go fullscreen, and when done I have to reattach the client(s). This for something that takes a command to adapt and again recover in screen.
It is just an inconvenience, nothing huge, but big enough to obstruct my flow.
Anything I do on my desktop that the mobile keyboard allows for (conveniently), I attach to the same sessions. But the most common one is IRC. And for that I still use screen just because of this issue (I always have IRC attached on several clients).
