Using varsity as an example, (which is a Div 1 club) - id say they are pretty damn talented if they fielded their reserve team (assumably their under 21s?) and only lose 3-2 to Western MPL. The premise behind splitting the Under 21 league up into Div 4 and Div 6 is because Mainland didnt want to go through the hassle of approaching, and then being denied by the Premier clubs in attempting to accomadate these teams into the PDL. When the organisation that aims to get the best players in the best leagues avoids doing so in fear of ruffling the feathers of the big clubs, you can argue that the Prems teams do have a fair amount say in what goes on with league structure.
I don't think it would have come down to asking the MPL clubs whether it was a good idea to change the PDL. I would have thoughts Mainland would have had the sense (?) not to break up an important comp such as the PDL to fit in a few extra teams. It was agreed that the PDL would be a mirror of MPL and this works.
Shame U21 didn't work, perhaps looking at the issues around this is a better use of time.