Who said it had to be serious? A sense of humor is always something I enjoy in products I use. a Unix system just isn't a Unix system without the bsd-games package! Silly easter eggs just make programs more fun to use; it gives them a bit of personality.

So long as they don't get in the way of using the software, I don't see the harm.

What's a serious operating system? Unix was invented to play video games, the first version of MS-DOS was called "Quick and Dirty Operating System"... And who would name his system the "Incompatible Time-Sharing System", anyway?