Hi Roman...
Simply this: more experience using pointer/mouse-centric UI interfaces leads to better skills in designing them. If you are still mostly using the keyboard, for instance, then you are missing out on many of the productive features of modern mouse-centric software.
If a person uses keyboard shortcuts instead of a mouse in software designed specifically to be used with a mouse (such as you claimed for yourself in your post), then that person
is less likely to have something to contribute to the design of a pointer/mouse-centric UI.
For just one example (of many), PSP (V8 and up) has one of the best implementations of multiple side-bar panels I have encountered (tabs, floats, docks, etc). If one doesn't use those features of PSP (which are primarily pointer/mouse-centric features), then one is less likely to see how such an interface design might apply to a TC panel (of folders, favorites, etc) or understand what parts of the design would work in a TC context, what parts wouldn't, etc.
Here is an analogy: If I was judging haute couture in Paris, I'd be looking at the models, not the dresses, right?
Which would be completely missing the point.
I am not trying to convert you (or others) to the mouse, or slow/cease the keyboard-centric design of TC! I just want a better TC for mouse-centric me!