What are we talking about, here? Having fun? I'm talking about social affirmation, collaboration, feedback, and community memory. There are a thousand social mechanisms for productivity. "Having fun" doesn't have to enter into it.

But it should! Humor is a valid form of communication, and it's my experience that the brightest people are the funniest—see: Tom. Lehrer. Smart teams wield levity among other tools. There is nothing virtuous about dourness or severity. There is a time and place for gravitas and decorum, but it's not 40 hours a week.

Besides all that, it's good for me! As you said, others may have different needs and, in general, humans are evolved to rely on one another to maintain psychohomeostasis. 40 hours is a long time to take me out of range of my evolved external organs.