Hydaelyn may not be all good. She IS Mother but where there is life there is death and where there is light there is darkness. That is why the crystals were once one.
I am wondering if the Ascians are attempting a Rejoining because they think it will create balance where there currently is none. They may not be all evil in their ultimate agenda. I mean Mr. Laughing-Bad (Lahabrea) was clearly evil and loved it, I respect him for that, but Elidibus seems willing to actually engage people in real conversations. He strikes me as someone who knows a whole lot about what's actually happening behind the curtain.
Which may be why Urianger is willing to listen to him