If the difficulty remains, the meaning of any party member in combat dwindles, and the friends of "6" always claim that the game has not to be rebalanced (while denying problems for the highest difficulty tier).
If the difficulty is unchanged, then it's important for a 6-person party to be an
option in settings, with a warning: "This game is balanced for a 4-person party. Your experience with 6 may vary." Then if people complain that the balance changes while playing with 6...who cares about their opinions? They were warned. Also, if the difficulty is unchanged, the meaning of each party member in combat remains the same (or increases). Combats will proceed faster because now the entire party is 1.5x as strong (more actually, because synergies -> "whole is more than the sum of its parts").
If the difficulty is changed for 6-person parties, then last part of your claim is moot. However, yes, in this case the relative effectiveness of each party member won't be the same as in a 4-person party. E.g., if each encounter has 1.5x as many enemies (or each enemy has 1.5x stats) then each party member effectively becomes ~1.5x less effective in combat. Although, again, you have to account for additional synergies between the 6-person-party, which increases overall (and individual) effectiveness.
