Tom says there's no excuse for adding easter eggs. I think this post of mine has generated more activity than anything I've written about WinFS or Longhorn ;)
I'm not going to advocate that every developer ought be able to spend as much time as they want checking in hidden features willy nilly. But I think there is something to be said for allowing developers to add some personal expression and creativity to a product that they spend years building.
The original Mac had the signatures of all the designers on the interior shielding of the case. To me, that shows pride, and passion, for the product they worked on. At their best, easter eggs serve the same purpose (although admittedly adding code to software is higher risk than carving your name into a piece of shielding).
I also strongly disagree with the notion that developers ought to be spending every minute of their time at work writing code. While that's clearly their #1 job, if you hired only developers who sit at their desk all day writing code (and don't take time out to experiment, or be creative, or de-stress at the foosbal table, or whatever), I think your product would be worse off.
[update: fixed spelling of easter in the first sentence ;]