That's not the case with me. I'm currently using a formation with 2 DMC's and they rarely score yet they have high ratings usually. When they are not performing good I usually swap one for a CM. In his case today the other DC was having a fantastic match with an 8.6 rating (no goal or assist). The same I tend to see with a AML ML AMC player. He usually plays better as an AMC.
I agree with your point that DMC get more involved. So many times during matches you dominate the GK and the defenders will have lower ratings (because they didn't gave to do anything).
I still think this is an interesting factor to look for.