Well, this idea is good, personally i prefer having the restriction of not being able to switch job/class once you enter the dungeon, the mobs are dumbed down by stacking certain classes that theyre weak to, just as an example in Aurum Vale having Blm's to firaga everything makes the run smooth and easy, the coincutter is weak against magic so at that moment nobody switch classes/jobs then you go all the way down to the boss entrance, then everyone switch to Mnk to beat the Bayhone in a very easy way, but wouldnt work as good if you had pure Blm's party, so having the restriction of not switching jobs would bring the need of several roles inside the dungeon, like the buffer, the tank the Aoe DD, the sub tank, the healer, etc.
Same happens in other dungeons like cutters cry were you switch from War to Blm in the last boss, by restricting the switch job system SE could make better challenges, so in some parts of the dungeons mobs might be resistant to magic but weak against physical and in other parts the opposite, so at the end you would need a balanced party were every job has an important role.
Well this are my 2 cents, take it or leave and it isnt a single side view