KDE 4 has all the benefits of Unity as well as the ability to be customized. For users that like a 'start' menu, you can have it, but you're right that keyboard launchers are just better most of the time. KDE has two built-in, and every other DE works with Synapse, which is much faster than Unity for keyboard-based launching.
The UI is optimized for tablets and phones, but not because of space efficiency. In fact, Unity is less space efficient, because it demands that the launcher be so huge (to afford touch-based interaction). In every DE I've ever used, I actually hide the top/bottom panel, but that's not possible in Unity (the top panel affords no customization whatsoever, AFAICT). So, in practice, Unity is consistently less space-efficient with my vertical space than every other DE, in addition to being less customizable.