DMC/MC is not optimal, no. Because you're combining a defensive position with a neutral one which renders few grey skills. But sometimes you need versatility before anything else. Especially if you're meeting multiple managers with varying tactics and formations. When I was active I found myself needing to switch between formations requiring 3 DCs + 2 DMCs + 2 MCs in the morning and DL/R + ML/R + AML/R in the evening. In that case, you require versatility above anything else. And usually I'd keep a very strong mutant offensive line, which would allow me to experiment with the backfield.
What I always did was having at least:
DC x2
DC+DMC
DL/R
ML/R+AML/R
ST+AMC
MC x2
Other than the above combinations, I developed players and positions depending on opposition during the season. I was never fond of DC+DL/R which you mentioned, as I don't like combining center and flank positions.