From what I understand, it used to be that if a couple had a 'serious' reason for using the rhythm method, they HAD to get their diocesan priest's permission, which required a discussion of the situation so that the priest could determine how 'serious' the reason was. If permission was granted, it was then determined "for how long?" because the Church never intended this to be a long term permission, but only for emergencies.
Nowadays, the rhythm method has turned into the full-blown, immoral, "NFP lifestyle" choice by the couple, with no permission from the priest required, and no obligation to God for their actions, and no intention of ever stopping. The exception has, literally, become the rule. May God have mercy on those couples who think they are blameless "because the Church said it was ok".