I was looking at it from a cost and marketability viewpoint.
Assuming that the CPL holder had to pay for everything, which seems par for the course in Europe and what you have to plan for, it seemed that 1/2 a MPL(A) course might be cheaper than an indpendant IR, MCC & Type Rating, plus if the MPL was 'sponsored' then you might be better placed to get a job.
Accepting that this may fix you in the RH seat for some considerable time may not be an issue.
Again, i suspect the cost would likely be about equal -- or even less for the IR. The 100+ hours of 737 or A320 sim time would not be cheaper than an equivalent amount of IR instruction. Same for the ground training for the "1/2 MPL." After that, a type rating would take a good deal less sim time (31 for my 744 and 37.5 for 742) -- and many airlines do not even require a type rating for F/Os (dunno what kind of rating comes with the MPL)! Add to that the fact that most/all airlines pay for the required aircraft-specific training for new-hires, which would be the same (or less) ground school and sim time required for a type rating.
Also, a "sponsored" MPL will likely incur an extended payback contract, with attendant lower wages very possible.
Marketability is another issue. Again, I have no doubt that an ATPL is more widely marketable than an MPL. Same with a CPL/IR. While a few airlines may give preference to those who complete their own MPL courses (and from whom they have already taken a considerable amount of money), that would not extend in general to other airlines. So, you would be trapped in the right seat of a single airline, with little or no ability to even try another airline, for a LONG time. You might be "better placed" to get a job with the sponsoring airline of an MPL course, but that would be the full extent of any advantage.
Finally, the ONLY reason airlines are going to the MPL is that they cannot find qualified CPL/IR holders, and it is cheaper for them to run an MPL course than to bring someone all the way from ground zero through a CPL/IR. So, any aitline that is thinking of hiring MPL pilots would also be prone to hire CPL/IR pilots. Those CPL/IR pilots would also get into the left seat WELL before their MPL contemporaries.
So, bottom line to me is that a CPL holder would be MUCH better off to complete his IR and look for a job with those credentials.