And it has actually turned out to be useful, besides being a lot of fun to work on.
I am not saying fun projects need to be useful (I'd be the last one to say that), I'm saying "why ensure they'll never be useful?".
Here's another language that is a 'fun project' that, who knows, might be quite useful some day:
http://wiki.reia-lang.org/wiki/Reia_Programming_Language
Edit and the classic "hobby project":
> "I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones.
Once again, by not tying a rock to his leg, it eventually went on to be just a bit more important than a hobby project.