Malta Flyer,
An MPL holder could get a JAA PPL quite easily. They'd probably just need to do a few hours training and take a check ride. They could then take a standard 25 hour JAA CPL course and test flight, to obtain a CPL. They could hire GA aircraft for solo flying with the PPL. They could get paid to fly GA aircraft as a single pilot with the CPL. They could add an ME with a standard 6 hour course.
MPL is not hugely different than fATPL. It spends little time focusing on building competence for single pilot GA aircraft and instead focuses earlier on building competency in multi-pilot ops on turbine engine types. The main concern is therefore, the smaller number of hours an MPL student gets in the air and as PIC, since the multi-pilot training is done in sims where two students can sit front seat. The second concern is the amount of sim time will result in less air sense and air experience. I would think that by 1500 hours, MPL pilots will have all of this, but they will be leaps and bounds ahead of fATPL holders on a Type Rating course.
If they ever do find themselves seeking a second job before they get to 1500 hours, they will have to take more training, probably to get the PPL and then CPL (they'll have an IR and probably a turbine ME already). This would give them an fATPL, but due to their high multi-crew sim time and type rating, they will be extremely competent in an airline sim assessment.