I don't agree with you here. Of course, you are way more experienced than I am, but for me, on the contrary, second DC helps more than second DMC. I mean, formations with three defenders (3w especially) are weaker in defence than formations with 4 defenders, but they give more posession.
I think the number of defenders by itself has a big influence on the game between simulations. 3w-2n-() may be a deep and nice looking defence formation, but for the game mechanics it is still 3 defenders and 2 midfielders, and it's not that good for your defending efforts than four defenders and DMC. Well, at least it's from my experience.