In my opinion, how long the connection from MDMA lasts post-trip is entirely up to the user. The first time I rolled, I felt that I connected/bonded with my brother very deeply. The next day, I was still ecstatic about the changes in me but, the day after that, the feelings started to fade. I became very depressed. My brother assured me that these feelings are just neurochemical in nature, which made me feel even worse because I felt that things actually had changed between my brother and I, or at least the potential for change was there. I decided that this potential need not go to waste, and I changed. I tried to be closer and kinder to my brother and more present in his life.
My impression is that MDMA creates potential for change. Some people will never be affected permanently by MDMA because they think that the effects of drugs are "just chemical" and thus cannot change personality. In my experience, these are the same types of people who experience grand things while on psychedelics but claim to have never had a spiritual experience. To such people, every drug-induced experience is a lie and nothing more. MDMA does not increase one's love for another person, but it instead clears away all of the BS that has clouded ones sense of love over the years. It is sometimes hard to realize our love for another person because our own personal ambitions, vices, and egos get in the way. MDMA clears the air until there is nothing left but you and that other person and your love for each other. Holding that love at the center of ones attention after the effects of MDMA has subsided is key in preventing the BS from getting in the way again.
Anyways, I think that MDMA is great for couples whose relationship is starting to fail because it does not just increase affection: It also induces understanding (or perhaps just the willingness to understand) and such understanding, if harnessed and applied after the drug has worn off, could save a declining relationship.
Do I think that the enhanced sense of love and understanding caused by MDMA lasts forever? No. But if you can stretch it out for a few weeks to a month, I think you'll do more good for the relationship than just about any couples therapy could do.