Friday drinks, anyone? We already do permit 'selected drug use'. Caffeine, Nicotine, prescription drugs (within limits). It's just a case of where do you draw the line. Currently, when it comes to alcohol, different companies draw the line on different sides.
Companies I have worked for have generally allowed it, however, they kept a close eye on it, however, the culture within the company was not one of accepting excessive consumption. They didn't hesitate to send someone home if they turned up for work hungover.
And I guess the core of it is; what is the company culture? Do you have employees who will hang around until midnight and drink everything they can? Or are they the type of people who will have 1 or two, eat a bit and then go home?
With regards to why it is necessary - it isn't necessary at all, but that's no reason to ban it. I think sometimes that workplaces are becoming about only doing what is necessary, and anything that doesn't directly relate to work that may invite a small amount of risk gets cancelled.