I dunno, they at least made it sound like the other girl has some sort of saintly power as well. If that other girl is completely useless, they would've just let her be the prince's toy while making Sei do all the saint work.
The prince was probably just trying to save face when he claimed the other girl could heal them but he didn't want to show her "such a scene".
So far we have been given no reason to believe this other summoned person has any powers besides seduction.
@LordoftheKeep well yea, to them, sandwiches and coffee are like...these radically new things. she's effectively like the time traveler who went back to the middle ages and brought a gun to them XD