One not-unreasonable explanation is that drinking together builds trust among co-workers, making them jointly more productive (and higher paid) despite the occasional productivity-reducing hangover.*
Radio New Zealand provides a nice bit of anecdotal evidence.
Josef Stalin and Winston Churchill had an all-night drinking session in Moscow that lasted until 3am during World War II.
Relations between the Russian and British leaders were stiff until Churchill arranged a late-night banquet in August 1942 with Stalin, according to files held by Britain's National Archives.
The event was recorded by Foreign Office permanent under-secretary Sir Alexander Cadogan.
The mood was "merry as a marriage-bell," he added, though Churchill was complaining of a "slight headache" when Cadogan came to find him at 1am.
The two men did not engage in much military talk during the meeting, which went on until 3am.
The evening was dubbed a success by Cadogan, as the two men got on.
"Certainly Winston was impressed, and I think the feeling was reciprocated," he wrote in a letter.
"We broke up soon after 3(am), giving me just time to get back to the hotel, pack, and leave for the aerodrome at 4.15(am)."
The BBC reports the letter was among almost 600 government files dating from World War II and the early years of the Cold War, released by the National Archives.
Do all-nighters really end at 3am?
I wonder whether the anti-alcohol activists of 1942 would have bemoaned the wartime losses caused by Churchill's slight hangover the next day while ignoring that getting on well with the Soviets was rather more important.
* Note that Google Insights for Search provides reasonable evidence that hangovers are concentrated on the weekend. I mean, look at this and tell me that the folks who take average weekly hangovers as a measure of productivity losses aren't just a bit mischievous.