If I have 4 players, I literally can't take a single origin character in my party. Period. They refuse.
1. No Lae'zel interrogation of Zorru 2. No Lae'zel at Gith Patrol option 3. No Wyll at windmill 4. No Shadowheart reveal 5. No Wyll with Spike 6. No real chances for increasing relationships with origin characters 7. No origin interaction at all except at camp, and it's very limited
6 member party size allows for 2 other companions, thus allowing for all of these things.
And in single player mode, I would have more companion interaction and dialogues outside of camp without constantly having to switch characters in and out.