It's not necessary to have the markup in a textarea on the demo page either when someone could just view the source and copy+paste it from there. But it's not about what's necessary, it's about what will be of utility to people.

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.