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It uses CoffeeScript's grammar, so there's no need to learn a new one or switch between two different grammars as you work.

It's also way less code than Haml. CoffeeKup's src folder is 240 lines of code.

I think the main advantage is it doesn't require Haml. I use Haml whenever possible, but I could see this being very useful for Node.js or other JS/CS-based apps.

why couldn't one write a haml parser for node.js? That would seem like a better strategy than invent a whole new syntax for markup.

There is a HAML parser. http://howtonode.org/haml-for-javascript

But it is a good thing to experiment with different approaches.

Not sure what you mean here-- it's raw CoffeeScript syntax. The templates it produces are JavaScript functions.