If you don't know JavaScript, you're really handicapped using any form of higher-level language wrapper. Coffeescript is syntax sugar. Run into problems or library incompatibility with other components and you don't know JavaScript...well...
BTW, for the record, I've never actually known a coffeescript dev who didn't know JavaScript, too, though.
What happens when there's a problem at the interface between your favorite compiles-to-javascript language and javascript? If you don't know javascript, how are you going to debug and/or fix that? I've never used coffeescript or clojurescript, so maybe they're so thin that this never happens?
