I say no, mainly because other classes aren't allow to switch Jobs mid-dungeon either, so what makes BLM/SMN, WHM/SCH and/or PLD/WAR so special that they deserver this kind of special treatment?

Other than that I'd have to say only because SE has this special system where you can max all classes on one char, doesn't mean it has to be abused to the max. You enter a dungeon on class, you finish it on the same class. Where would you draw the line for switching jobs? Like you open a loot chest as a BLM and find a SMN weapon so you just switch to SMN to be able to need over the other players? It would also simplify the game as you could always change to the job that's better suited for the next pull and/or boss, instead of having to make do with what your initla group composition offered.