I wouldn't want to call MotoBlurMotoBlur or SenseHTC Sense just "skins" -- they aren't a suite of visual enhancements, but an entire app suite that sitessits on top of the OS. They're full-fledged graphical user interfaces with additional custom apps, virtual keyboards, widgets, etc.
So, as to the benefits? I like HTC senseSense - its my major UI on the Incredible, but I've also used my wife's Aria. The Sense UI is cleaner and smoother (hardware speeds aside). The custom widgets are smoothly integrated and have more features. Other than the HTC mail and messaging apps having a possible memory leak issue [I replaced their use with K9 and Gmail, and Hancent]Handcent], I've been pleased with Sense. A smooth, consistent user experience with the interface can make using it better. It's not the be-all end-all -- despite the recent MS WinMo7 commercials -- but it's nice.