so that it is functional prior to you completing the feature. There are MANY things missing from that UI. Like a main menu button... thats a pretty core
If something as far reaching as multiple plains isn't being tested in sync with the rest of the game at a stage where they're testing pretty much anything else in general gameplay then that sounds to me a bit like they're halfassing it. I think it's a pretty core mechanic and will have wide ramifications for the whole game so would need to be attached from very early on to come out well.
Also I still don't understand why you'd have a UI without a plane switching button? I mean what are you testing is funcitonal here with this mock up that leaves it out? Are you just checking the a button press leads to a particular feature working? If so then you could do that just as easily with more buttons? If you're talking design and looking for an intuitive, visually clean, efficient to use interface then it just strikes me there's no point testing a version of it that has no chance being in the game as it stands. Surely in that aspect you're checking that it works at the micro level, that everything is exactly in the right place and optimal for use... but why do that when you're necessarily gonna have to add in another button and shift everything around, it invalidates any testing you would have done
.. but having said that I could easily be wrong, maybe there's great reasons for doing it that I don't comprehend.. I am not a game programmer 
I reckon the bottom left button on the current screenshots could conceivably be a main menu button (the one that just has a crown on it) .. it's also possible that could be some sort of character page for your channeller..but I think it's generic looking enough that it could be for the menu.
To make it really intuitive it would be an easy access hotkey on the keyboard that lets you switch planes without moving your mouse (solves my problems with old planes system again)
Yeah if multiple planes are in they DEFINITELY need a hotkey for switching, and you're right it really does solve a lot of problems by allowing you to put your cursor somewhere and then flicking so you can see where the planes match up.. yeah that's really an elegant way of doing it. Having said that of course I wouldn't call something that relied SOLELY on a hotkey "intuitive" as to me that implies being able to figure out how to use functionality with no prior knowledge.. and unless you're blindly experiementing by mashing buttons on your keyboard that'd have to be explicitly told to you somewhere and not implied by the interface which is what we surely want. Also there is the fact that a lot of players like to play with only their mouse, so incorporating an important feature that forces them to be in reach of their keyboarrds at all time seems a bit mean 
Man I can't believe I wrote so much on such a minor speculatey point.... I.. am.. an... idiot! 