Programs should be fun to use. But by far their most important reason for existing is that they are fun to write. If dropping in an easter egg allows a hacker to blow off some steam and consequently stick around the office for a few hours longer, and put in a 20 hour day instead of merely a 16 hour day, then those are resources well spent.
Professionalism has no place in art, and hacking is art.
I'm a hacker, not an engineer. Convenient though it would be if it were true, Mozilla is not big because it's full of useless crap. Mozilla is big because your needs are big. Your needs are big because the Internet is big. But being a shining jewel of perfection was not a goal when we wrote Mozilla.