I have many, MANY redirects (thousands). Most just redirect the system (node/12345) url to the friendly one - but we also have a lot of others.
I have Drupal set to never delete old redirects after they haven't been accessed for X amount of time, but the mere presence of the option to do so makes me think there's a legitimate reason I may want to.
Is there a lot of overhead? Or is it simply to keep the list manageable? Do people typically delete them, and if so, what's the time frame? Six months? A year?