Aside for any personal preference, a sudden realization just hit me a couple of hours ago while checking the "six-men mod" (which is actually more a save edit than a proper mod).
Why is Larian NOT taking advantage of the fact that this is an Early Access build, precisely to give us an option at the beginning of a new game to pick between the "standard and supported" party of 4 and an "experimental and unstable" six characters mode"? No rebalance or tweaking of any sort required. Just that, an experimental option.
That would give them a chance to see what people choose, what they tend to stick with, how much they enjoy it, etc. All through client-integrated telemetry. Isn't that what a EA is supposed to be about? Or is it more that they just can't be bothered to try it regardless of what people want, at some point?