What if a player doesn't give a flying fuck about Larian's way of doing things? What if said player just wants to play for laughs? Oh but wait, you wont get your little Achievement badge saying what rough, tough adventurer you are. I have run around Skyrim with 70 followers in tow. The game is designed for 2 if I recall correctly.
Depends on how the game deals with XP but increasing the party size could result in slower level progression.