It's not that it can't work. It certainly can, as was shown with the introduction of jobs in 1.0
The actual problem is that it worked so well that it threw off the balance and made PLD undesirable for many weeks.
I believe it's better to simply implement a job that will change the role of MRD, but being based off of it. For instance, Berserker to MRD could be like SCH to ACN.