Should SQL Server MDF/LDF files always be on separate physical disks, even on databases that are barely used?
Solution 1:
If there is no real load on the databases I would have them co-exist with another database LUN set. It's always best to keep the database and logs separate and since you are already doing that for other databases, keep the same scheme.
Solution 2:
I would at least put the ldf and mdf files together on a different drive from C: That way if the server fails you won't lose the data.