Second, loose couplings are great for those who want to tweak to their heart's content, people who know what they're doing. I used to tweak the heck out of Gnome 2. I grew tired of it and appreciate the simplicity and elegance of Unity and dislike the "we know what's best for you" attitude that Gnome 3 conveys to me. Unity just needs to have 3 sane defaults reverted that I've mentioned in other comments.
Third, you can still tweak Linux to use whatever desktop you want. Back when I was on Windows I even hacked the registry to make the DOS command prompt my UI as a joke and it worked. Booted, no explorer running at all. Just the Windows XP kernel, few processes it started and DOS CLI that was it. It taught me that most of the stuff users interacted with on XP was actually started by and run by explorer which explained why the system grew so unstable when it crashed and I had to restart explorer.exe from the Task Manager by hitting Ctrl_Alt_Delete IIRC.
Having a common UI/UX is great for a lot of people out there including those of us who know all about loose coupling and tweaking things but usually just want to do what we want to do without all of that tweaking. Except on those weird days on the week-end when we try new stuff out like a checking out how KDE is coming along. Still too complex for my tastes but it looks great!