I think the thing people aren't taking into consideration regarding "min-maxing" your party is that Larian is balancing this game for a 4 person party, so this should become way less of a concern. They will likely have to consider things like alternative tactics and strategies for specific fights due to the 4 party limit and the possibility you won't have X class with X spell/ability. Like D:OS2 I'm sure you'll be able to mod in a larger party, but just like D:OS2 it'll likely make the game completely trivial in every encounter due to the nature of the game balance.
Neverwinter Nights was balanced around having a party of 2 (yourself, and a henchmen), it doesn't change the fact that me trying to complete the Beggar's Nest in Chapter 1 as a rogue, with Tomi Undergallows (another rogue), was impossible; Instead, I was effectively forced to take with me a different henchmen for that district, despite the fact that I A) enjoyed playing a single-classed rogue, and B) liked Tomi as a travelling companion (because of his quips and conversations).
So no, I don't particularly feel less concerned just because Larian says that they are "balancing the game for a 4 person party". Maybe, in BG3, I want have a party of 4 fighters (no casters or healers or skill-monkeys) or 4 warlocks (almost no spell slots among the four of them, and no healing capabilities). Increasing the party baseline to 6 would mean that even in situations where you have all party members being the same class, you can generally make up for it, by overcompensating in other areas (and without Min-Maxing).
EDIT: Also, seeing the previews of gameplay, it looks like alot of their "balanced for a 4 person party" also requires that the player not be a casual gamer, but master tactician. I personally don't want to spend 5 minutes tactically placing my party members before each fight. Maybe, just maybe, I would rather just breeze through combat so that I could enjoy the actual PLOT of the game. A party of 6 could also allow that to happen.
