Think of an RPG game. Zoro = offensive power while Marco = defensive. Now who would you rather have in your team?
For me, defensive is more important than offensive. Take basketball game for example, you have to make sure that nobody get the ball, point guard runs the offense and usually is the team's best dribbler and passer. The point guard then defends the opponent's point guard and tries to steal the ball. You can play defensively and offensively at the same time.
Point is that Zoro would be an amazing offensive player while Marco is a better defensive player because of his healing ability. And you don't need Armament in order to necessary hurt Kaido, you just need to counter Kaido's scales and you will be fine. Marco should have some way to damage Kaido just like how he damages Big Mom's Homies.
This is stupid on so many fucking levels
Zoro = Best offensive player you can get
Goal = Killer the opponent that can not be killed and would tank all your attacks
Zoro = Very good at defense as he saved Luffy's ass 3 times already
Marco = Good defensive player
This is not a damn basketball game but still I would want offensive player in NBA game as well. But in this particular situation, goal is to take the head off of Kaido and defeat him and Zoro is best at doing that. Zoro is not a support player, he is a lead offensive player and lead player is more important than support player.
Marco would be a very good support player so you should be comparing him to Law, Kidd and Killer, the three who are irrelevant as they are but best they can do is support Luffy and Zoro. In that regard yes Marco would be more important than Kidd, Killer and Law.