1. Reread Wano. You'd be surprised at how little of a bond Luffy and Yamato share. He didn't even mention her here
2. And? She didn't fight alongside the Strawhats at all
3. At this point, pretty much. Zoro and Usopp had their flashbacks early on in the series before long flashbacks became standard
4. So she's able to turn into a four-legged furry mammal? If only we had another Strawhat who fit that description...
5. Whatever you say
But yes, having a role is an absolute must. After all, Luffy has
- The world's best shipwright
- The world's best navigator
- (Soon to be) The world's best sniper
- (Soon to be) The world's best swordsman
- One of the world's best archeologists
- (Soon to be) The world's best doctor
- A world renowned rockstar
- One of the world's best helmsmen
- The world’s best chef
You wanna know why Carrot didn't join despite all of the hype surrounding her? It's because she didn't have a role
Same deal with Yamato