What if I don't have MM. Should I still go to DL on a MM day? Will it be already extremely crowded when I enter?
Definitely start at DL on a MM day even if you don't have MM. Only Fantasyland and some Tomorrowland rides are open, so you can still get an equal rope drop for the Adventureland/Frontierland side of the park. Then later in the day, hop over to DCA if you want. If you are only going for 1 day, try to avoid MM days; however, multiple day visitors should not.
If I don't have MM, how can I use MM for my advantage?
On MM days, DL has specific gates designated for MM and others that say "Entrance". Those who have MM can enter through the MM gates all the way up until official park opening. Those without MM usually line up at the gates marked "Entrance". HOWEVER, if you arrive early enough and wait by the gates marked "MM", oftentimes some of those MM gates will turn into Entrance gates as the time approaches official park opening. If you do it right, when a MM gate turns into an Entrance gate, you can actually be one of the first ones in line at a gate.