Why not add it? It's not a standard as in browsers won't use it, but haml is probably one of the most widely-used html templating systems.
I used Haml in the past, and I understand it's popular nowadays mostly because of Rails, but I hardly think it's widely-used. Just to name a few that use HTML or a template engine of their own: Blogspot, Tumblr and Wordpress, and these are much more widely-used than Haml.
Consider the audience of Bootstrap when you talk about usage numbers. I'd wager the number of people using Bootstrap + haml (either through Rails or standalone) in their apps outnumbers the ones using Bootstrap + Wordpress/Blogspot/Tumblr.