There may be some number of couple-carpoolers who would otherwise have taken two cars, but I would have to guess that's a tiny minority. Given the goal -- getting cars off the road -- allowing couples to be carpools is a very poor way of doing it. Which is more "fair" -- letting 99 couples who were already carpooling into the lane (at a cost to other users of the lane, and unfairly to all the other people not allowed in) and not allowing 1 truly carpooling couple, or the other way around?

Of course in places where 3 is a Carpool this is much less of an issue.

And the other issue is that it's almost impossible to enforce a rule declaring couples not to be a carpool. So in spite of the fact that it would be better, it's not really practical. In the reverse, not allowing kidpooling, because it removes very few cars from the road makes things easier to enforce, as you no longer have all the cars that look solo but have a baby in the back.