Does it make sense to answer old questions in case other people stumble across them? For instance: Custom Post Type Archives with 0 Posts Redirects as 404 (fixed in 3.2). I always figure you never know where people are looking for information :)
You spread the good and up to date knowledge around.
You help reduce the nasty unanswered pile.
You get rep and possibly nifty Revival or Necromancer badge.
Also don't forget the Google.
Old Q/A's certainly show up in long tail searches.