The way I understand it, QML allows you to design way more advanced and customized widgets than the basic widgets that come built-in with Qt. In their case, there were a lot of things they wanted to do that wasn't possible/available with standard widgets. An really the result is stunning.
However, and that's an important point, musescore has been taken over by a company. There is now a professional team spending a lot of time on it. Their need for QML came also from the fact that they could spend a lot of time building design documents, preparing things, planning, building guidelines, testing, etc.
Also QML, contrarily to standard qt widgets, needs to be coded. No more graphical editor (as far as I know). This makes sense when only a handful developers will deal with that anyway. But when you have a community-developed software, like FreeCAD, using maybe less "shiny" features in favour of more widely, user-friendly, easily handed tools like standard Qt widgets with Qt designer, makes a lot of sense too. Same with our wiki. We could use shinier, kick-ass documentation systems. But we'd loose the easy entry that the wiki provides, and we would not have so many people wanting to contribute.
As Ton Roosendaal (father of Blender) says, your real asset is your community, not your software. I like that idea a lot. And BTW by looking at how Blender gets developed, it's still a community-developed project, although it has serious money pouring in. And notice that although they do work on the UI, they are absolutely not making it a nice shiny, polished, unified thing. People still call it an airplane cockpit
That said, of course there is a lot to learn and grab in the musescore case. We absolutely could do more "design" in FreeCAD (specially the Qt/coin interaction), and think of developing our own widgets where we need it (And with theming too it's possible to produce kick-ass results).