My counterpoint to these arguments is that less is more.
With 6 characters you can cover all bases, use all gear, easily optimize your tactics, see through most companions stories to completion, etc.
Having only 4 makes choosing your party have much more tangible implications and makes many choices more meaningful. If you can't fill every conceivable party role or have every spell you might need, tactics matter a lot more than if you had the expansive breadth of options you get with 6 PCs worth of tools.
You can do this with your 4 character party. But let others the bigger party for more variety / opportunities / fun. I can easily reverse your reasoning. Precisely because there are several characters with different classes etc., one can give foes the "coup de grace" in several ways. With more Characters you can have more "backgrounds" for more background goals. If implemented well into the game, you will addionally have to make tough decisions. For example the fisherman at ravaged beach. If you have a folk hero & a criminal then you have to choose what to do. This could be nice for a further alignment / reputation system and maybe combined with some deity buffs or similary stuff (e. g. unique abilities for classes or from artifacts / weapons like in Tyranny).
