'need' is a bad argument, a game is a system with a goal and rules, the correct way to play the game is to try and achieve that goal the best way you are able to within the rules. You 'could' just meander around and be less effective, but nobody is going to say thats how you are supposed to play something like chess or football, yet they always bring that argument up when it comes to video games.

The current combat system could be summarizes as 'remove enemy actions while maximizing your own'. This means you try to do the most optimal combination of CC and outright killing so as to minimize the amount of stuff that can be done to your party over the course of the battle. Since to CC you need to break an armor first, CCing and killing both follow along the same strategy of simply loading damage and then either doing 'more damage' or 'apply a CC'. Both of these are done more efficiently with more options and more flexibility with a party that stacks the same damage type. A single support mage can indeed work in a 'physical' party just because you dont need 4 people worth of damage to handle most situations if you build and gear yourself apropriately, but that doesnt mean its optimal because:

Is that mage along for purely support skills? Anyone can get those support skills with a single point investment, you dont need a dedicated mage, everyone can be running armor of frost/fortify/haste on physical damage builds
Is that mage along for CC? It better be able to strip all the magic armor itself, first turn, but even then its better to be another physical character and just take warfare skills which also provide aoe cc but also help with killing. Your party also being physical means you can more reliably use your own CCs, and they can use their own, because its all synergystic
A fighter can take lore just as well as a mage can
Likewise in an all mage party you can just have one mage take thievery there is no inherent need to have a proper rogue.
Mages along for ranged CC? Makes sense at first, after all archers CC is more limited than warfare based character so maybe having a mage to cc stuff at range would be smart.. or you could just take 1 level of aero for teleport and teleport that archer next to the enemy melee and CC them all with warfare skills.. which is more optimal.

This is one of the issues actually, you dont need a smartymcsmartypants to ID stuff for you, anyone can take loremaster, you dont need mr sneakyjerk to pick locks, a mage can do that. In some systems you try to tic all the boxes in order to have the things you need in a well rounded adventuring party, and you end up with some characters doing different kinds of damage and maybe it doesnt all fit togeather, but its for a reason. But you can just optimize in this game to the point that these weaknessess can be built out of the party entierly, lowering the overal dynamic nature of the strategy involved. Everyone can support magic, do a single damage type, and do all the kinds of things you would traditionaly have had specialized 'skills' characters to do.

There is no way you can claim the different armor types do not encourage damage stacking. Im not being anti-magic here since i do think an all mage party would work as well, its just that currently its easier to stack warfare based characters.

Of course if each character can one-shot a single enemy, self contained, per turn, it doesnt matter which damage type they use. So ok, a party of 4 absolute offensive powerhouses can mix and match, but that throws the idea of a support mage out the window. But again, there is no reason to ever have a support mage, everyone can learn all the support skills they need with 10 int and a few point investments. The support mage is, itself, a sub optimal character that has no place in the current game becuase everyone can be their own support mage.