For me it's not about writing a GUI for the current client - there is already a GUI for the current client, so why to change it? Just so it would respond a few ms quicker on mouse clicks? Maybe that would be a good reason if it was slow, but it isn't slow. The biggest problem of the current GUI is that it's a hassle to change anything in it and hard to control/review the changes.

I thought wx was being abandoned to make further development of bitcoin's UI easier - that would be a good reason.
Unfortunately IMO you won't make further GUI development easier by switching from wx to Qt.

Maybe the GUI shall be indeed a separate application which talks to bitcoind via some interface - like the JSON-RPC, but with both-way notifications, so the GUI doesn't need to pull for data.
Then you could create your OS-specific GUI, while I'd do my web based one - and everybody would be happy