You make good points, but from my own experience the times easter eggs get added to software are often related to underlying tech changes.

Consider that the fireplace screen saver might not just be about having a fireplace, but perhaps a test of streaming video over the air, or maybe it's rendered with OpenGL, or maybe it's testing a new app deployment mechanism, etc. Easter eggs are a great way to test new tech that could be very important, on a low-impact area of the system.

Or the fart mode could just be fart mode.