I know the guys behind this project.
And as far as I know PlainJS is all about performance.
I've seen a lot of projects including megabytes of libraries. So a litte bit of purism is maybe not a bad Idea.
But I think this is just a starting point. In my opinion they need to provide test for there lib..
It's not just about performance. That can be important for complex projects, such as image editors, but it's not why I personally prefer plain JS over jQuery.
However, ask yourself: What's your advantage of using jQuery today? Modern JS is pretty cross browser safe and syntax is becoming simpler and simpler. Modern browsers, like Chrome or FF, have already DOM4 implemented. With that, a lot of jQuery-like methods were accepted: .prepend(), .closest() ... querySelectorAll() is essetially the same es $(...) in jQuery.
Personally, I was surprised how easy and effective it is to write vanilla JS. Which is why the idea of plainJS.com came up. And as a proof of concept, plainJS is naturally fully implemented in plain JS :D And it wasn't hard, and it works properly down to IE 8.
