What happens when you give HAML to a non-Rails designer? Maybe it's just taste, but I feel that true markup with template tags for data (erb) is much better than a whole new markup language for designers to learn. YMMV.
I think "designers" or non-Rails people are only the tip of the iceberg though. I've been doing Ruby and Rails since 2004 and while I've tried Haml a couple of times, it never clicked with me because I already both knew and liked HTML. It felt like learning Japanese for the fun of it, yet I can already say everything I want to in English.
Regardless - A designer that can design for any* framework with standard CSS/HTML is more flexible than a designer that knows HAML, but could't write vanilla markup.
That why I was surprised about the Coffeescript (preferring HAML is a popular opinion, as-is Rspec: yet Erb and Test::Unit are still the defaults).
Edit: I agree that alt. languages/syntaxes in frameworks are nice. Forcing them on people is not nice.
This is a very bizarre statment. How can someone write haml without understanding HTML and CSS? How could that even be possible? Haml is little more than classes, ids and element names.
In my experience as a Ruby contractor I've seem many, many more WTFs in erb than haml, to the point where when I join a project using erb I now brace myself.
Edit: a note, I am plenty comfortable writing vanilla HTML, I just prefer the speed and clarity of haml.
i.e.:
:textile
Hi, my name is *#{@user.name}*!