I think that problems tend to arise when people are using a free bar because they order spirits or expensive drinks. We had a party for our 40th and arranged lunch in a local restaurant with drinks provided. There was champagne to start before the meal and wine at the tables but no ordering their own drinks. This kept the cost to a reasonable level, as did the fact that the meal had a set finishing time rather than going on all afternoon.
Of course you also need to know that the restaurant is trustworthy and won't charge you for bottles that have not been used.
Desperately sad story of the assisted suicide of a grieving mother


