Many of these "weekend" projects turn into long-term projects. The thing that gets launched initially helps the developer guage interest before he spends too much time on the project.
I'd have a hard time believing that 'many' of these weekend projects go on to become a long-term project.
To me at least, they seem like people taking a shotgun approach to starting projects and hoping that one of them will stick and become popular without having had to expend much effort on it. Interesting problems take more than a weekend to solve.
