I don’t consider Yamato’s “specialness“ to be much of an argument against her.
Jinbei, for example, is the only one who was at Marineford, the only one to watch Ace die, and the only once who saw Luffy have his total breakdown where he questioned becoming the Pirate King. He was the one to then snap him out of that. That certainly makes him special among the Strawhats, but he still joined.
I’d say that if Yamato is to join, sticking in her Ace relationship and knowledge of what Luffy‘s true dream would be a balance to her being absent for 80% of the series and thus missing so many moments.
Jinbei, for example, is the only one who was at Marineford, the only one to watch Ace die, and the only once who saw Luffy have his total breakdown where he questioned becoming the Pirate King. He was the one to then snap him out of that. That certainly makes him special among the Strawhats, but he still joined.
I’d say that if Yamato is to join, sticking in her Ace relationship and knowledge of what Luffy‘s true dream would be a balance to her being absent for 80% of the series and thus missing so many moments.