Ian

Bounty isn't just about strength. It's about how high a priority the World Government puts on catching you. Robin is definitely a Commander-level priority given that she is the last survivor of Ohara and her ability to read poneglyphs. But she is not on a quest to be the strongest and hers is not (primarily) a combat role.

That said, the Strawhats do need more Haki users and Nico is a good choice, Armament and Observation make sense for her character. Franky gaining Armament to augment his cyborg parts, and Chopper gaining Observation to better diagnose and triage patients and to augment his empathetic bedside manner would also not be amiss.