I don't know. I'm not sold on the idea of living apart, but I can imagine a scenario where it makes sense to keep two households, or two homes. I am 47. Let's say I meet a woman close to my age who also has her own place, things, etc. I'm not going to start a family at my age. And, why should one of us, or both of us, liquidate and then get a place together? Why not keep both places, mine in the country and hers in the city (that's how I'm imagining it at the moment, anyway). We can stay at her place some, mine some. Maybe some days we stay in our own places. Personally, I would not have a problem with a situation like that.
Also, people work all of their lives to have something to pass on to their children. When mature people get together, things like that need to be taken into consideration. I would not expect her to sell what she has and move in with me if she has grown children who would have been given land or a house or what-have-you.
I agree that married couples living apart doesn't seem quite right. But, depending on the circumstances, it may be wiser for a mature couple to not put all of their eggs in the same basket. Just a thought.