Dean was sort of getting a feel for who he might end up in a foxhole with. It was something he'd prefer to do, given the chance, with any hunter he didn't know. He needed to know Ellen could handle herself, and that he wouldn't be saving her ass, and could trust her with his life.
This place wasn't going easy on anyone. And if the things that were happening with Jo were as bad as he though they were, Ellen was going to be needed.
And if she was bored, Hell, that just told him that she'd want in, even without ending up here outside of her own devices.