I don't understand what screen size has to do with it. I understand that these ideas might have come into vogue through the development of mobile UIs, but the inception is irrelevant to me. I would think this was a better UI even without the example of mobile touchscreens.
When I want to launch a program, or I'm just thinking about what to launch, I want to see a big interface that shows me all of my programs at once. And I like the feeling of "ownership" that I have in rearranging them how I want.
I'm probably a visual person. I know not everyone is like this (particularly not the average left-brained geek), but I like having a clean, visual representation of options. Having things hidden away: in fractions of my screen, in nested folders, etc, actually interferes with my workflow! I don't know what my programs are, I can't really see them, I forget they're even there... I spend a lot of time trying to imagine what apps I have when I really just want to SEE them.
Being able to call up a visual representation of my app options actually speeds up my workflow.
As opposed to the start menu where everything is permantly hidden... No "clashing" to speak of because you never even have visual elements in the first place! It's all just lists of text with microscopic icons.
You can call these differing styles a clash if you want. If they really bother you, there are ways to skin old icons with new design. But personally I don't care. The old icon serves as a reminder that it's going to open in a desktop window anyway.
The point is that this is better to use. I push a button on my keyboard and I have everything I actually use on my PC grouped how I want it, in a visual manner. Nested folders/drop down menus are bad for launching programs.