The Problem with Phoenix is that is has really low stats. So only in a case of "I got a Fluffy for myself" or not having a other 40% Eidolon, you could consider her. She is the 2nd worse Light Eido to date, so if you are playing longer chances are you dont even use her in Sub aswell cause besides Sakura, every other Eidolon is better (yes, even Lilim).

If you are not having a 40% Eido for Thunder, it could be considered taking her in for that Element aswell. Otherwise, you probably just gonna feed her into the Eido Shop for good.