While I'm not sure taping and cataloguing every episode of Letterman indicates that Mark Hamill himself has an obsessive compulsion with the show, for others their fandom definitely crosses a line. Letterman also had a female stalker, and that wouldn't have happened without his nightly appearances on TV. That woman definitely had a problem.
I think that it is very possible that an extreme fan of something or someone could become obsessed to a point that it is unhealthy. While there is no harm in liking something and participating in a fandom, it is important to remember that your real life is more important.
Yes, there is a point when fandom becomes an obsessive compulsion that should be treated. If it comes to a point where the fan is stalking or frightening the celebrity, this is an issue. Also, if it comes to a point where the individual can no longer function in his or her everyday life, this is definitely an issue that needs to be treated.
Mark Hamill's habit of taping and cataloguing every single episode of Late Night with David Letterman is not the type of activity that needs to be treated. Now, if his obsession were to spiral into other areas of his life and damage relationships or Hamill himself in the same way that an addiction to drugs or alcohol would, that's another story. But simple fandom - regardless of how apparently obsessive - does not warrant treatment.