Yamato’s character that exploded in popularity as soon as she was introduced. Every now and then you will see a new character introduced into One Piece that a ton of fans immediately fall in love with; Yamato’s one of those characters on steroids. Her connections with Kaidou & Ace, her design along with her devil fruit and combat ability all makes her very easily to like. On top of all of that, she’s one of the few people who directly asked to join the crew. So with all of this, why would people think she’s not joining?

Her fruit is the “Guardian” of Wano, so she must stay in Wano.

It’s important to remember that we got that information from Kaidou, who has everything to gain from Yamato staying in Wano as shogun. Yamato has spent her entire 20 year imprisonment rebelling against Kaidou. Even when Kaidou gets defeated and driven out of the country, Yamato’s staying in Wano to protect it is essentially fulfilling part of Kaidou’s goals.

In order to fix Oden’s mistake, she must stay to protect Wano

Yamato’s calling herself Oden and living like him is in part because she feels like he represents what she lacks; freedom. Her emulating Oden doesn’t mean she’s obligated to fix all of the mistakes Oden made. However, the problems Oden caused by leaving are what are being address in the arc as I type this. Orochi & Kaidou’s control of Wano represent Oden’s biggest mistake, and by helping drive them out, Yamato will solve the problems caused by Oden’s departure, staying on Wano after the fact won’t help fix anymore significant problems caused by Oden.

Yamato joining doesn’t fit the formula followed by previous Straw Hats

The process of a new crew mate joining hasn’t followed a traditional pattern in One Piece for well over a decade. If how new members join the crew was as set in stone as other aspects of the series, like Luffy, Zoro & Sanji fighting the top 3 members of opposing crews, then It would make sense to hold Yamato to that formula. That said, the last two members to join the crew didn’t join in the same fashion as prior members, which sets a precedent for newer members to also break that mold.

She’s too strong to join the crew/If she joins it will ruin the Monter Trio dynamic

Robin’s words on Zoro & Sanji being the wings of the Pirate King should be enough to stop people from thinking the M3 dynamic is going to disappear. The whole M3 dynamic has evolved beyond just being the three strongest members. Zoro & Sanji have been shown to provide important moments of leadership when needed on top of fighting the strongest opposing crew members. That said, Yamato joining does not mess up the dynamic. It’s important to keep in mind who the Straw Hats will be facing off against.

The SH’s will essentially be a fully fledged Yonkou crew following Wano, meaning that they should be strong enough to clash with any other Yonkou crew of even the World Government. This means that the Straw Hats should have a minimum of 3 members outside of Luffy who are capable of clashing with an Admiral.

Yamato joining will take attention away from other Straw Hats

I disagree. Mainly because I feel like Yamato’s being used as a scapegoat for issue’s caused by other things/characters.

Post-Timeskip, most Straw Hats have lacked attention/development combat wise. This is primarily due to Oda’s tendency to introduce new characters into each arc that he feels need character arcs of there own that need focus. In Dressrosa, the various number of colosseum combatants that were given screen time took away from a potential fight for Robin. Every arc has a certain number of side characters or arc specific characters that sometimes wind up taking priority over individual Straw Hats (outside of Luffy) for an arc, this is more of a problem than an individual joining the crew.

She’s interacting with Momo more than Luffy

Yamato and Momo have too many parallels for them not to have a connection. That said, the connection they have is specifically relevant to the current issues on Wano. As stated before, once Kaidou and Orochi are gone, nothing beyond personal choice is keeping Yamato on Wano. It’s also important to remember that most of Yamato’s major interactions with Momo have occurred because Luffy requested she help him.

She hasn’t interacted with any of the other crew mates

Oda’s been very intention in how he’s had Yamato nearly meet some of the other SH’s, just to have something get in the way or have them run off in another direction. It happened with Robin & Brook, it also happened with Sanji. These scenes are such blatant teases, it makes it obvious to me that Oda wants the reader to be excited about when Yamato meets the rest of the SH’s. If anything, her not interacting with any of them outside of Luffy & Franky is Oda’s way of building up to her meeting the rest of them and joining the crew.

Oda loves to build up to big moments. Yamato meeting the SH’s is a short term example of him doing that.

There’s no clear role for her on the ship

The most obvious role for Yamato would be for her to do as Oden did and chronicle her adventures with the SH’s in a Journal. Given her attachment to Oden, it only makes sense for her do the same.

Luffy doesn’t remember her name, he doesn’t give crew mates nicknames

IIRC, Luffy didn’t refer to Franky as such until he joined the crew, pretty sure it was the same with Chopper as well. That said, I don’t think this is anywhere near as important as people make it out to be. Luffy’s focus right now is taking down Kaidou. Given how simple minded Luffy is, him not remembering Yamato’s name is likely due to his focus being elsewhere. Depending on how Wano ends, Luffy’s focus shifting away from their immediate problem will likely be all it takes for him to call Yamato by her name and take her request to join seriously.

In conclusion

In general, most people’s arguments are motivated by what they want to happen. So most of the people arguing against Yamato joining feel as though she shouldn’t and vice versa. I think it’s very easy call people biased and attack them rather than their reasoning.