I actually did not yet see any real argument why a party of 6 would be more fun.
The only reason I can imagine is that then you can have 1 of everything in the party. But that for me personally would ruin the fun of having to choose whom I bring with myself onto a quest, and also lower the replay value of the game.
And most arguments are like: BG1 had 6...
The argument is that as long as the game balances (like through XP sharing), what is the issue of you playing with 4 and me with 6? BG1 & 2 were with 6, so yes nostalgia plays a part, but also for more choice in party composition. Whatever your pick, there are enough arguments in here for. Other than "it's too much effort for Larian", I have yet to see a good argument against. Surely it's no skin off anyone's nose if people want to play with more characters in the party?
Of course should the party lock mechanic rear it's head at the end of Act1, then yes, it could be interesting what happens if you select only 3 extra characters vs my 5... Do you actually have to selecct 5 and keep 2 in camp? Is there a camp? But that is the same discussion for those who choose to play Solo.