I’ve written about link rot quite a few times (it even has a category on my blog).
Preventing it is important, as it improves user experience.
For most users this is an unconscious thing when it works and becomes consciously annoying when it fails.
Some user groups are vocal enough to force you to fix link rot after the fact, causing brand reputation damage.
One good example was last year: [Wayback] Users condemn Microsoft for removing KB IDs from some bug documentation | Computerworld.
The Wiert Corner - irregular stream of stuff 