I use a frontend because I need to be able to handle music and GPS (and once in a while, small apps like calculator, etc) while driving. Personally, I don't find navigating winamp,etc to be touchscreen-friendly enough to be safe while driving. And switching between apps is easier when I can just press the big "back" button to get to the main menu, and then push the big buttons to get to music, GPS, etc.
Also, I much prefer the look of a frontend to a windows desktop, it looks much more integrated with the car.
As everyone says, clearly it's personal preference; surely you could optimize your apps for easy touchscreen use (skins, etc) but it seems to me that by the time you do that, it's going to look more and more like a frontend anyway.