I think the reason why people elect to play in the slower events is that the timer starts ticking as soon as your opponent moves.

So, if you have any type of vacation planned in the months ahead, you have to make sure you have enough time to make each move as if you were actually on your vacation or away from your computer, "just in case" the game spills into your vacation time.

With the N moves in M days approach, you would not have to worry about this at all. Look at the 10 moves/30 days time control. The game starts, you go on vacation for 3 weeks, using up 21 days, and you still have 9 days to make 10 moves when you return. You will not time out.

What would be your alternative here and now? Play 21 days per move.

A big difference.