My guess is that those Masses are the ones that typically have MCs. If a parish has MCs for simpler Masses then maybe it’s kind of practicing for more complicated ones.For big Masses, particularly with visiting clergy they can certainly make sense.
True. But often these groups don’t train together. When priests con-celebrate it’s often because one is a visitor. If you have large numbers of lay persons and several priests and deacons then you get the cases where the priest is waiting for a reader who is waiting for an altar server who is waiting for the deacon who is waiting for the the cantor who is waiting for the user… An MC can discretely tell the reader do whatever even without all the others doing whatever they should have been doing. That usually cues the lagging persons to do their jobs.An MC is no substitute for well trained celebrants, con-celebrants, deacons and laypersons.
In my opinion an MC is something like a director for a play. A liturgy is a bit like a play or a dance in that timing can make the difference in whether a liturgy seems jerky or beautiful. When the MC is doing his/her job right then it seems like they are doing nothing.