As others have said, it's all fun and games until two people edit the same source file, one of whom has the dangerously incorrect opinion that tabs are better than spaces :-)

I use a lot of semantic whitespace languages (CS, Haskell, Python), and it's a tradeoff: you gain in terms of redundancy, true, but there's an additional human overhead of making sure everyone's got their editors set up correctly. The larger the team, and a fortiori the number of teams, the bigger that overhead is. Smacking people about the head with PEP8 goes quite close to solving the problem in the Python world, but CS's Rubyish commitment to TIMTOWTDI militates against that.

"It's like SASS vs. SCSS. Or Haml vs. ERB. I can't, for the life of me figure out why anyone would prefer more to less."

Because HTML and CSS, despite their verbosity, are linguae francae. A CSS file is valid SCSS by definition - a less technical prototyper person can give you their code (perhaps even generated code from some wysiwig monstrosity) and you can refactor over it to remove redundancy, enforce conventions etc using the special SASS sauce. The SASS syntax does not.

A HTML file, likewise, is already an ERB file. HAML is 'backwards compatible' with ERB/plain HTML, so far as I recall, but mixing the two syntaxes is very, very ugly and a dreadful idea, and I'm not sure if it's possible to mix the two in similar template languages (Jade etc).

Point is: there is a human factor involved in picking shinier toys. I like HAML/Jade type templates, and Coffeescript for that matter, but I am far more comfortable using them on small teams or solo projects.