The first law of UI design is that you will fuck up most of the time. Most of your ideas are going to be bad bad bad. If you think that every mock comes out of your workstation smelling like roses, then you're either deluding yourself or you're just knocking out something incredibly derivative. To be a good UI designer you have to say no all the time, and to all of your ideas.
So here's the problem. Static image mocks only paint about 60% of the picture when you're talking about modern (especially touch-based) interfaces. Static mocks don't give you enough information to accurately cull your ideas. Sometimes you need to build the damn thing, so you can touch it and move it and realize well damn, that wasn't such a great idea after all (or oh, this part really needs to be changed or the final, magical yeah, that's not too bad).
All of the best and most innovative UI designers that I've met have been able to build interactive mocks of their designs. Now - don't ask how they did it. It's spaghetti code that will curl your hair, frequently written using some arcane technology that might not even be supported anymore. But it works just well enough that they can use it to iterate on their ideas.
Now, you can split this job into two people. One a "designer" and one an "interactive mock engineer". In theory, it could work, but in practice I've found that it becomes hard for the designer to detect the flaws in the design. I'm not sure why. But the final product doesn't change as much. It doesn't evolve as much. It's rarely as good as it should be.