Dian is needed if you're planning to run a vigor gun grid with thunder using mammon gun and guardian guns. She's excellent to keep your health top off for that bonus damage. That said, pulling anything with jewels is a crap shoot at best, so chances are pretty low of getting her even on rate up. Then again, Taphro isn't gonna come back with jewels as far as we know, only chance for her would be with nutaku gold early next year or via anniversary mtix.

IMO, the more jewels you save, the more disappointed you'll be when you spend them. Might as well use them now.