Look. Party of 6 just plain makes sense all the way around. Single player or multiplayer both.
Single. Create 1 PC and have all 5 origin characters in the party. Instead of XP share, where everyone gets 300 XP because the MC got 300, divide by 6 and everyone gets 50, leveling up slower because bigger party. Want to earn more XP, only take party of four. That's your choice.
Multiplayer. Can create up to four PCs and still have room for two other party members, so you can take someone like Lae'zel or Shadowheart in your party to trigger those story scenes that require them in your party to trigger. If you don't have six available slots for party members you can't take any of the origin characters with you if you play with three other people. Therefore, in order to make multiplayer work, you need to be able to have a party of six so you can have four players and two origin characters at a time.
This is a silly debate because the way the game is built 6 party members is the only way it works for all scenarios.