If I leave the office stimulated and excited about what I'm working on, I will work until 2 or 3am in a frenzy, and get a lot of work done.
If I leave the office tired and exhausted from hours and hours of yak shaving, I go home and rest.
Everybody wins on the 20% days, especially if they're friday, because I will likely spend the weekend coding what I didn't finish friday night.