Custom formats are great when you want to keep your info locked up, (like game graphics, etc.) but they won't do you a whole lot of good when you are creating a paint program for example
Well, this is a game programming language, so it's a fair assumption that one is making a game using it. As for me, I like to make random modules that're neat (mostly so I'm not bored) and then share them.
Anyhow, that's a different subject.