You guys do remember that Zoans users are specialized in physical attributes, right? Their actual endurance is far greater than most other characters in the series, outside of those who have abnormally large willpower.
Another thing it makes sense separating luffy away from the ongoing chaos , Yamato might play important role helping him to navigate the onigashima also the historic nature of this island . There is too much into this plot and we are really far away from actual war.
If Yamato allies with luffy that could boast up the alliance and i am pretty sure he has loyalist with him . Yet the plot pretty much shows he is improtant for crucial big mom - kaido alliance party .