I always have found 6 character so much chaos and confusion and don't understand why people are nursing about rules that much. In my pretty party design, i ll create a frontline warrior paladinlike, a healer wizard enchanter or cleric, a rogue of course in different shapeways and a guy with very devourer destruction skills. But 4 character is very limited in a rpg fantasy world ı can see that. 5 is the best in my opinion, you can add whatever you want far from any limitation, and no such chaos as 6.
Seems quite the arbitrary line to draw in the sand.
Still, I'd be fine with five as a lukewarm compromise. Six is still the superior option by a landslide. Not to mention it makes party formation symmetric (which totally counts) and it's the only option among the three that implicitly allow the other two to exist without any particular issue.