I think Otama should at most be a temporary member like dog/cat was to Roger. I mean, battles are much too dangerous for her, and they already have Nami/Usopp to protect. Even so, Nami/Usopp still have deadly manoeuvres up their sleeves.
In my opinion, Otama has two characteristics that make her too broken for being a permanent straw hat crew.
1. She is completely vulnerable to kidnappings (like Mokuba in Yu-Gi-Oh).
2. Her fruit power is too broken. If she was on board, it would completely rewrite the battle trio's objectives. The smartest goal would be to try to get Otama's face-riceball into the enemy's mouth and automatically win instead of kicking the enemy's ass until they faint.