Perhaps I am dumb, but it seems so strongly related to salary and gameshape that you may as well just look at salary and gameshape.
dmi could be more powerful, because gameshape have subs which aren't displayed directly and salary could increase during the season.
So overall dmi could help you find out which player is currently better then another for the game(if they had comparable strength and weaknesses), but you shouldn't make a buying decsion based on dmi because it is so much affected by gameshape.