Why is it not backwards compatible? Nothing the user does would changed. I doubt there are people out there with pickled exposure variables, but we could add a warning. 

This PR either 1) fixes the bug its for only 2) Goes all the way and solve 3-4 outstanding, related issues or 3) Solves 1.5 issues and adds a hack for checking exposure in models where we don't necessarily need to.

Right now it's 3, but it'd make more sense to do 1 or 2 I think. In any case, we've talked about this PR more than it warrants.