I don't think they care about the pope's endorsement. I think they want the photo op so that somewhere down the road they can posture themselves as serious religious leaders who engage in substantive dialogue with other serious religious (and world) leaders, instead of a bunch of grads of marginal right-wing Bible colleges.

It's also possible they want to be able to say (and here's the risk for Francis) that they reached out in good faith, but that mean old pope was unreasonable as always. As I said in another post, Francis shouldn't take the bait.