The issue is that with the M0 lockout, as you go through the dungeons, your selection as for which dungeon to do starts to shrink.
Compared to spamming +2, you can pick any dungeon, and run them as often as you can get a group. For many, most of the time, any dungeon will do, as they’re typically farming for currency or vault slots. Even moreso, folks may want to focus a dungeon for a particular drop. With M0 you get one crack at it per week.
If M0 were in group finder (is it? I don’t remember), then, like heroics, there would be no lockouts. That is, heroic do have a lockout (daily? still?). But if you do a random dungeon, the lockout doesn’t apply.
If M0 were like that, they’d be swarmed with toons queueing for them. That would make them “useful” heroics.