Unity is not non-free. The problem is most users have NVidia/ATI graphics and
the free drivers don't have any/good enough 3d acceleration for it. You
should be able to run Unity with Intel graphics though and free drivers since
they have 3d acceleration support.
While I'm not saying MATE does not have an active development community I
think Cinnamon is where Linux Mint is going. That was the impression I got
from Clem (the lead developer of that project) when I spoke with him a short
while back.
I think Linux Mint has potential and while it isn't a free distribution today
Clem doesn't seem to be hostile to freedom either. Lets keep working on
Trisquel and one day Linux Mint will have a free version or being only free
software. It's where I'd like to think GNU/Linux in general is going. We are
certainly going to push it wherever feasible (mostly on the back end, free
drivers, free firmware, OGG Theora based video streaming services, etc).
