When designing a UI, it should be compulsory to: Get an evil IT guy to draw up a humongous list of tasks that need to be done (find all files created on a certain date, copy all the pictures from the last picture session but not those before that, delete every file beginning with A, run X programs and switch between them) and then - when your UI designer submits a design, make him sit through two months of 9-5 work doing those tasks.
And every time the UI designer admits that something is wrong, he has to go back, redesign, and do another two months. And every time that the UI designer is not significantly faster or at least on a par with said evil IT guy doing it ANY OTHER WAY THEY WANT, it's back to the drawing board again. And next to the UI designer, there's a little old granny. And she has a set target of doing the same tasks but at some portion of the speed a pro can do it in. Every time the experiment resets, the granny is replaced with a new one. And the UI designer has to sit in the room and answer all their questions, help whenever they ask, repeat how it all works all over again, and generally tolerate them trying to do the same things that everyone else has to do.
And this shall be re-done every time a new device type, input method, display technology or whatever else is required occurs. Touch. Multi-touch. Gestures. Desktop. Portable. Large-screen. HD. 4K. Whatever. Start all over again.
The UI designers would quit if they had to talk an old lady through a mail merge from the point of turning on a new computer for the first time. It'd take them hours just to get to a desktop past all the tutorial shite, let alone into office, into the right menus (that fold up when you're not looking) and through the "wizard" which appears on the right and is apparently invisible if you're over 50.
Eat your own dog-food.