Without a rework of the encounters the highest difficulty would be a joke for the party of 6 players. They (or some) would accuse Larian to be a liar about the proposed masochistic "Way of the uber Damned", being a cakewalk in the game. Or not?
If the only change to the game were that a party of 6 could be used, why not.
It could also lead to some interesting forum talk later, about the option, it's users, about self restriction and why you can change it/cannot change it during playthroughs for example.