Worked at several flight schools, owned one, and am now wandering the world in a whale.
What bothers me about the MPL is the lack of solo time, especially cross country solo time. This seems to me to be where novice pilots learn to make complex decisions and implement them. All of my students seemed to learn more on their first few solo trips then in myriad training flights.
As to the value of PA28 time, just took a 747-100 into FJR, before any of the navaids were up. With no procedures in place and no guidance from ATC, we ended up doing a (quite large) light aircraft landing pattern.
When I have flown with ab initio graduates, they have seemed uncomfortable outside of the standard ILS, or maybe RNAV approach.
In spite of my prejudice, I expect that MPL graduates will do just fine for the most part... I am pretty sure that my ex DC-8 captains are not to impressed with my skills, and probably their ex DC-3 captains were likewise unimpressed, as were the open cockpit graduates before them...