Marco is here because there are two S class enemies and they don't have any allies above A level (Luffy so far). The enemies have a good number of like , B~D tiers so I don't think any of the unnamed and obscure allies will be worth a damn here. Without a Marco on the battlefield it's hard to see them making it in good condition to face those monsters and even then they'll need more contribution to the final battle.
Zoro has nothing else to really do. He can whittle down the fodder and chip in on an emperor, but that means this arc doesn't make any progress on his goal. If chipping at Kaido was a real achievement then Enma would already be black. Beating King alone sets Zoro up at A class while fighting the emperors prepares Luffy for S class
