In fact I have written more than one DIY static site generators for small sites in the distant past. The bottom line of my experience was: they don't worth the effort. You will start missing features sooner than you think, and it will be entirely up to you to implement them from scratch. Instead of actually adding content to your site, you'll end up adding features to your feature-strapped hack.

In the presence of excellent tools like Jekyll or Pelican that are also fairly easy to use and extend, I'd need a concrete use-case before going DIY. If there is no concrete use-case, the DIY static generators is just another version of the "not invented here" syndrome. Just my 2¢.