Yamato’s dream has been made painstakingly clear over the last few chapters, including this one - she wants to be free from her father’s oppressive clutches and experience the world as her idol Oden did. If you say this isn’t strong enough motivation for her to join, I’d argue that Carrot’s dream is the exact same thing, so either both of them have good reason to follow Luffy, or neither of them do.
As far as the role, I’ve long said that someone doesn’t NEED a position to justify traveling on a ship. Zoro himself is a combatant, which isn’t a well-defined job, and although they mean something to Luffy, you don’t really need an archaeologist (Robin) or a musician (Brook) on a pirate crew. There are only so many tangible jobs, and yet, we have seen plenty of crews with far more bodies than would reasonably fill them. Oden himself didn’t seem to have an actual responsibility in Roger’s crew, and filling the role of commander for WB also doesn’t carry any requirements past being powerful. If we’re gonna sit here and pretend like “lookout” is a necessary role, tell me why we’re 1000 chapters in and the SHP have gotten along just fine without one.