The Github wiki is pretty bad. However had, it is easier to use than issues and coordinating them. Github issues require too much cross-communication and not everyone has the time to meta-coordinate many different issues in many different projects. A wiki lowers the entry-barrier too, so the idea of a wiki is, in principle, good.

If I were Github I would improve the wiki, a bit stylistically, to make it more visually pleasing to use (but not much, those designers always go overboard when making changes in my experience), but much more importantly so, to make the wiki a more flexible addition, including API-wise, usage examples, documentation and so forth. People can, in principle, do so on their own, but also from experience, most people stop doing so after a while, and then the wiki decays into outdated information. That's bad too. Spawning more issues to manage the wiki also does not work well.