No, its simply like how in XI you can make any combo of main/sub you want, but for common sense you just don't because most of them are inefficient. Some have their places though. And so what if there is a healer that is not as good at healing but can hit harder? thats like a WHM using a light staff in XI vs a whm using a DPS club. They'll each be a bit better at certain things (granted that game lets you gear swap for magic/WS/abilities)
So the game lets you do it, but out of common sense you just wont unless for specific reasons. XI doesnt have an influx of DRG/SMN running around, even though you can.