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 canâ€™t change stuff on the go). Iâ€™d be baffled if we wonâ€™t encounter situations where we need* to have an arcane caster, for example. Likewise, i canâ€™t imagine to play with no front line without abusing/cheesing the AI.
* â€œNeedâ€ meaning not in the sense that you canâ€™t complete the game, but meaning that, should you not have one in your party, the resolution/outcome would be miserable, you canâ€™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).
Seems unlikely you ever need any particular class to achieve something. Giving players multiple ways to approach things is one of their main design philosophies. Having the right class might give you an easier option though.