I believe DMI is calculated from skill levels (including stamina) and gameshape, and sublevels in those do matter. That's why DMI can help you notice differences in sublevels in gameshape. For example, if you player has 1 million DMI at gameshape 9, and the next week he his gameshape 8 with 950.000 DMI you know he's a high 8 for gameshape.
As to how to evaluate who is a better player at this moment, you'll need a combination factors. In isolation you can focus on the individual skill levels (including stamina and free throws, and not all skills are equally valuable) combined with gameshape. However, when deciding who to put where in your lineup, there's also the factors of offensive and defensive tactics, matchups, team composition which can influence the decision. There's not a single number that helps you make this decision. If you don't have the experience to make this complete evaluation, DMI can be a somewhat helpful shortcut to it I think. However, you need to put some extra emphasis on secondary skills, as they are not reflected properly in DMI.