I have more than 6500 hrs TT, 3000 PIC medium jets (A320/737). I have an ATPL, naturally. I did a CAA 'approved' course (200hrs) and some private PPL flying before I got my first job as a jet FO.
What exactly is it that you think I learnt in the 51 hours of solo flying (circuit-bashing, hour-building navexes, PFLs) which I had before I earned my command, that the MPL pilots have missed out on? Or maybe you think that hours of flying manually round the hold was good preparation for commanding a modern jet, 'cause I did that too.
I think the best (i.e. relevant, interesting, useful) bit of my route to a CPL was the 50 hours of MCC training, post-IR, with a mate in a jet sim. Only then did it all make sense.
I am not worried about the ability of MPL holders to fly an aircraft, more that the MPL scheme seems rather airline-specific, and therefore keeps pilots almost in hock to their 'parent' airline - not a good thing for industry terms and conditions.