The reason why it can be done in frontlines is because there are no party build restrictions at all (which is something many people are actually complaining about now, due to the fact that some people have been making healer-dominant groups and pretty much being undefeatable and stuff)

It's possible that they are using frontline to test some sort of mid-duty job switching mechanic, but somehow I don't think that's a high-priority thing for them right now...