My dad worked for a rather large insurance company. One of the hacks printed the user's name, marched it around the border of the terminal (thin client to the mainframe) and then deleted itself before giving the user their prompt back.
Yep worked for a pretty large company and they had a (capable), high ranking dev guy who was prone to add easter eggs in the UI. Konami code would cause something to fly across the screen etc. The live product was taken down twice due to these easter eggs.
The amount of complexity, bugs, and security vulnerabilities in modern software is already at a staggering level purely due to non-easter-eggs. Not only will allowing the developers to occasionally have some fun help them to be more engaged (and produce higher-quality software as a result), but a well-implemented easter egg touches a highly localized portion of the system and won't introduce any of the above in the first place.