I don't really see a thing wrong with couples living separately, and it shouldn't be yardstick for their respective happiness. Basically, happiness is something we own ourselves, it shouldn't be something dependent on any other person, be it one's partner or anybody else. Often than not people find themselves disappointed because of the high expectations of happiness from others.
Back to the matter, if couples find it deem cool to live apart from one another, they should give it a try. Besides too circulation of commodity makes it less valuable. Same it is when couples are at each others face every now then. You'll notice that that respect and affection that was once flowing from both ends will wane a little. But with little space every now and then between the couples the relationship will greatly be spiced up with much needed missed feelings.
On the other hand, if the couples think they can always live under the same roof, they can go on but it shouldn't be the primary reason of happiness for both of them.