i'm sure it's been mentioned but one downside of a party of 6 is that you'd have to add more enemies to keep balance, making combat take even longer. In a RTWP game this is a non-issue because combat is so fast to the point you need to add trash encounters. But in turned based, it can make things feel really sluggish. I already feel some encounters in BG3 are way too slow when too many creatures are involved (i.e. Shattered Sanctuary).
This has been mentioned, yes. The counter argument has also been mentioned, namely that BG3 could use divided experience. If you divide exp among the characters in the party, then a party of 6 levels up more slowly than a party of 4, and the game mostly auto balances itself.
Edit: Also, people are arguing for the option of playing with a party of 6, not a requirement or expectation. Encounters would still be based around party sizes of 4; it's not like we want the game to check in real time what our party size is, and then adjust # of enemies depending on whether we have 4, 6, or 1 characters. Divided exp also allows players to use party sizes smaller than 4, where characters would level up faster!