We actually had an interesting situation where I work (spacecraft operations). We had a senior aerospace engineer depart after 15 years to become an airline pilot, of all things (decided to turn a hobby into a job).
About a year later, he came back part time because the routes he flew left him with large blocks of free time at irregular periods during the month, and he was getting bored (because before his "hobby" was flying....and he stopped doing that on his days off!).
It was a win-win situation. He'd give us 40-60 hours a month of hourly work when it was convenient for him. We kept his hopper full of things like documentation, training, and other stuff that most senior guys consider dreg work. Even though he now has enough seniority to avoid pilot furloughs, he'll volunteer to drop his flight hours if the airline needs him to. He just increases his hours with us (and he's so good, we'll take whatever he gives us up to full time).
Since he's not interested in advancing up the ladder, he really does a great job on this low-visibility stuff that really helps an organization run well if it's done right.