An operators conversion course is a JAR OPS subpart N requirement for any pilot changing from one airline to another, regardless of whether they hold an MPL, CPL or ATPL.
Thus an MPL holder if they had to change airline immediately after completion of their type rating is in no worse a situation from a regulatory point of view as someone holding a traditional "frozen ATPL".
Most objections to the MPL seem to come from those with a vested interest in it not happening, such as instructors who don't want to be instructors and fear they will not be able to move on to the airlines, and from the FTOs which are not big enough to have the facilities to offer the course.
I am sure that the training and flight ops departments at BA, Flybe, Monarch, Easy etc will have considered this very carefully and would not be going down this route if they thought they would have problems in a few years time because captains on a multi-crew airliner who had trained under the MPL system didn't have 100 PIC on a light single in VFR as part of their initial training three, five or ten years previously.
The fact that the MPL doesn't have single pilot priveleges should be advantageous eventually for those who want to instruct or fly single crew operations, as they will not find hundreds (thousands?) of airline wannabees taking those jobs to build hours to move on. It should also meant that salaries for aerial work and single crew public transport jobs might increase as the pool of pilots would be smaller.