Scriptable favicons can be useful and unobtrusive when used judiciously. For example, adding a small "1" in a red circle when a new message arrives. Sure, you could just replace the favicon with another image that has the number hard-coded into it, but that's a bit clunky and you'd need to prepare several different images.
When is the last time a web feature with the potential to annoy was used only judiciously?
I now expect the use of tabs to become impractical as half the websites on the planet start animating their favicons and I can't handle the distracting movement in the corner of my eye.
What web feature doesn't have the potential to annoy?
(Not the Tetris - scriptability of the Favicon.)