I think that's acceptable, yes.
There is actually one case of someone using a nonfree license for
nonfunctional data that I particularly don't like, and that's PlaneShift. The
PlaneShift license for the nonfunctional game data says that you can only use
the game data with a client distributed by the PlaneShift developers on their
servers. I don't like this because it's completely inappropriate for a game
data license, a backhanded tactic to be restrictive while still being able to
boast that the software is "open source". In fact, they proudly claim in a
page that the purpose of this is to prevent forks:
http://www.planeshift.it/license.html
So I'm aware of one case where someone has managed to use nonfree
non-functional data to effectively restrict freedom, but as far as I know, it
is currently unique. Doom, for example, can be played with any free (or
nonfree, but why would you do that?) Doom engine. Similarly, nobody can stop
you from using AssaultCube's nonfunctional data when playing with whatever
modifications you like to the client on any server you wish.
