It comes down to grand plans of getting all the wildlings to do what we tell them to. Yes, they're a pain in the ass, but we need them to not be Beyond the Wall when the Long Night comes. A big part of that is going to be diplomancing each and every clan we come across and show them our strength and that we can fight the beasts of the Others, so when the time comes and we tell them to pack up and leave, they pack up and leave.
Diplomancing Elle is a part of that, it's not just a mad scramble for all PCs in sight. We diplomance Elle, that translates to diplomancing the crab people as a whole. And also, we fail to diplomance her and it basically turns our victory here into a failure because all she's going to talk about when she gets home is how we wouldn't even kill the bear-men that ate her tribesmen.
EDIT: And we've already seen that rumors can and will spread among the crab folk. We mess this up by setting Elle off and we have a permanent malus among the crab people.