DMI is not a very good way to measure how good a player is.
Of course DMI says nothing about how good a player is; still, DMI indicates fairly well how good a players' performance is at the moment. Overuse of good players degrades their game shape and makes them much less useful for the team (if retained), and somewhat less attractive for would-be buyers as well (if put for sale). Thus, if you want good return for investments in training and salary, keep their GS at "strong" at least, otherwise profits are gone. That's all I mean.