I really love writing code in bem style and really like Jade, so I though that adding some bem to Jade would be a great little thing that at least I would use a lot. And I hope that it would be helpful to someone else too.
If you hadn’t already read the articles on BEM on SM, or Nicolas Gallaghar’s article on front-end architecture, — go for it! Those articles explain some great ways to write extendable and mainainable code.
And Jade is the template engine heavily influenced by Haml by TJ Holowaychuk.