@King:
There was also nothing point to Robin joining when she was introduced in Whiskey Peak.
You are constantly making false correlations with past events. Your comparisons don't hold up.
Robin's role during the Alabasta saga has nothing in common with Yamato's role now.
And the narrative decisions behind Robin joining have nothing in common with the narrative reasons why Yamato is important now.
Robin is a totally different character from Yamato. To begin with, Robin was mysterious and full of hints of future developments. Everything about her, from past to motivations to future importance, was emphasized as a mystery, even in the PoV of other characters.
Meanwhile, Yamato is pretty plain and straigthforward exposition character.
I'm not even sure what revelation you expect from Yamato, since pretty much every mystery about him was undone by he himself in the most expository manner within the first 2 or 3 chapters he was in.
It's as if Robin revealed her Ohara backstory and interest in Poneglyphs when she first appeared in the Merry.
–- Update From New Post Merge ---
@legumes:
I'm really not sure how after yamato has stated their desire to go out + knows luffy's true dream + gets a lot of focus and little tidbits hinting at greater elaboration (aka a sizable flashback) anyone can really say this is a character that's only meant to be an arc character. None of the examples given have even shown to be characters that have lasting presence beyond their arcs.
Because the importance of any of that for a character becoming a protagonist is solely in your opinion.
Yamato wanting to go to sea does not equal he joining.
He knowing Luffy's dream doesn't mean he's tied to Luffy.
If anything, those things say more about Luffy being the Chosen One than Yamato being important beyond Wano.
People take whatever clues they feel like and blow them out of proportion, as if there's only one way of reaching resolution.