We did open bar. Some folks drank, some folks didn’t.
The inclusive price assures no surprises.
If you are going to pay the tab either way, open bar gives you a known set price. If you are going to pick up the tab anyway, and have them charge you by the drink you have no idea what your bar tab may be. You may wind up under the open bar price, or you may wind up way over it.
Lets face it, at the end of the day how do you prove they didn’t give out 500 drinks? Lot of room for some padding and fraud if you are paying the tab by the drink.