Ok then, I ll give some tips not only from my personal experience or observations but from the forum too.
Why I don't use DML - DMR
If oppo playing from the middle (4-3-3, 4-4-2 ND) , I prefer to use 3DC+ 1DMC, to have my power also in the middle.
If oppo playing with AML-AMR I prefer to use DL-DR to confront them as they 're the most dangerous players for scoring.
ML - MR are also very good specially if using red arrows.
So, I prefer to buy a DL-DC or DR-DC instead or DL-DML so to make rotation
Most of the times, when I 'm playing vs an opponent who using DML-DMR, I observe their stats. Usually they 're harmless for my defense.
Exception is some player from the scout list with positions DML-ML-AML (R). Those players can be very dangerous.
I would buy one of those but only to use him as a ML-AML.
About DCs : There is no need to have all the defense with double positions (DC-DR, DC-DL) because you always must use at least one DC, so don't give to your DC an extra position. If you have greens, better give an sa.
Finishing with defense, try to have a "Header" and a "Defensive wall" in your defense.