I will play the Devil's advocate here. On our planes there's that kind of open bar in Business and First Class. First Class passengers very seldom use it, they usually ask drinks to be taken to their seats. The bar is used more by Business Class people in need of a leg stretch, or a little snack (there are fruits, cakes and sandwiches too). Honestly, it's never happened on one of my flights, or that I've heard of, that someone took advantage the way someone fears they would. You should consider that the bar is always manned, because at least one CC is in the galley at all times and the 2 bars are one each side of the Business Galley. It's more likely that people come and ask for a coffee or an espresso. The times when I've witnessed someone drinking too much, they were always calling from their seats.
In Economy we have a similar bar policy, but only non-alcoholic drinks are on self-service, while for alcoholic drinks you have to ask a CC (but they are free anyway, we don't sell alcohol on AF).
I'd say the important thing here is that that kind of bar shouldn't be left unattended. It would be interesting to hear the experience of CC of other companies such as Virgin that have open bars.