it's the reason why people critisize barrelmancy, because "just don't use it" argument is bad, players who hate barrelmancy will feel forced to use in an encounter they find sufficiently hard. They will hate it, but they will do it, and chances are they won't enjoy it that way.
This is a valid complaint (#2 in my earlier list of "valid complaints") and I emphasize with this. Honestly, I think I'd enjoy playing BG3 with a party of 4 way more than with a party of 6; 6 is just too many for me to have fun handling especially with current BG3 movement mechanics, UI, etc. And if the game by-default allows a party of 6, I will either have less fun controlling 6 characters or I'll play with a party of 4 but feel like I'm missing out on story/banter/companion reactions to the world. Neither is great.
HOWEVER, a solution is to make a party of 6 an option in game settings: set to off by default and not advertised anywhere. Complete with a warning: "BG3 was created with a party of 4 in mind (or a party of 1 or 2 via Lone Wolf). Checking this box will allow you up to 6 party members but we make no promises about your experience." This'd be enough to convince my monkey brain that I'm not missing out by sticking with a party of 4.
And thus I still advocate for a party of 6 option, because - assuming it's implemented as above - it'd make a lot of people happy at ~minimal cost to myself. and also because a lot of people against a 6-person party are making bad arguments and I can't help but explain why said arguments are dumb