Just wondering what you thought.
If a single person goes to dinner at a friend's home, he/she takes a bottle of whatever it is they like to drink. Usually wine. It doesn't really matter if they only drink 1/3 of the bottle, they still take the whole bottle and, if they are normal, they don't take the rest of the bottle home with them.
So if you are a couple, you should take two bottles, shouldn't you? Why is it then that so many will take one bottle between them, because they are a 'unit'?
It was the reference to couples/singles paying for taxis that sparked this off - still not sure why couples think they should only pay for one fare, but that's another thread.