MPL or CPL you still cannot get a. ATPL without a skill test and 1500 of the right sort of hours.
However, I am certain that if I hadn't been a confident operator of light aircraft, challenging myself all alone as most PPLs do, then I would never have become the confident operator of the 737 that I now enjoy flying. These guys will not be truely tested until they are Captains, and then realise what it is like to be in an unusual situation, especially if all the F/O can say is "Errrm"
All things being equal, I am sure that a CPL ME/IR pilot with an MCC (JOCs are a waste of time and money) could transition to a mediuim jet not only just as easily as the same pilot undertaking an MPL course, but also be a more rouded and accomplished pilot throughout their career too.
Personally, I believe thaf a 250hr pilot who endeavours to always fly a different route to a different destination, pushes his comfort zone but is able to fly defensively whe required, has had a very rich 250hrs.
In terms of multi crew flying, an MPL course will be no more effective compared to a traditional MCC and type rating.