Its been brought up before but just to highlight that when there is a party of 6 players you also get more opportunities to play different equipment or spell loadouts. I think it'd be particularly nice for spells and arcane casters esp since it makes preparing utility spells more viable. Right now they really need to focus on dmg but if you have like another wizard or bard in the group you can have one dude wrecking fire balls and another with like abjuration or going illusion style.
With 6 there is more room to redupilcate class but with a different focus, after the core 4 the fifth and sixth slot is where the party variety comes into play.
The prob with everyone can do everything, is that you have to long rest and change loadouts constantly if you want to try something off key. So instead you just go with whatever is most OP for that archetype.
In BG1 by the time you got to the Friendly Arm inn you had Charname, plus Imoen and Montaron/Xzar pairing. The Khalid and Jaheira pairing came soon as you got to the Inn. These were cool because they were already grouped together. So you had the Harpers or Zhents choice pretty early and a reason sometimes to see one chunked with amusing one liners from the other companion.
There was just way more gameplay nuance involved in selecting a party composition and who to ditch or switch. In the Beregost area you could pick up a Bard or another Fighter. There was a Ranger or Cleric just outside town. By the time you reached the mines you had already encountered like a dozen possible companions. That's the vibe that's missing right now for me with only 4.