I agree with your points.
DMI is an expression of skill and game shape put into one metric. I don't find it very useful to compare players though because - similar to salary - it seems to be quite high for players with a few high skills and rather low for all-round players.
Players skills (as _wella_ described) usually only change for old or trained players. That means most changes in DMI correlate to the status of game shape and thus allow some estimation of game shape sub levels.
Personally, I use it only for my own players to see how close they are to lower or higher game shape levels when I want to decide who to start in the next game.
However, you can also apply this to opponent's players if you have a record of their DMI over time (buzzer-manager has it!).