This if fair, if we are speaking about tabletop campaign. However, in this case you have a DM that can tweak each encounter (either social or combat). And, to be honest, i do agree with you; i partecipated in campaign/oneshot where we played with no front line, full barbs party, etc.
That being said, we are not speaking about a live campaign but a pre-established one (where you can’t change stuff on the go). I’d be baffled if we won’t encounter situations where we need* to have an arcane caster, for example. Likewise, i can’t imagine to play with no front line without abusing/cheesing the AI.
* “Need” meaning not in the sense that you can’t complete the game, but meaning that, should you not have one in your party, the resolution/outcome would be miserable, you can’t access some area and so on.
At any rate, this is just another issue related to the party number limitation, but i already given up on that (sigh).
