Every time I've looked to hire one its disappointing that they're so un-rounded in their skillset.
If you're on the frontend I expect you to act as a bridge from the server to the user and have some feet in both worlds. At least being able to understand controller code, etc, but with the JS-wow hotness of the day there seems to be a desire to put a hard line on both sides and only deal with comps that a designer has given and and API docs from the backend dev. I don't think this works in any midsized or smaller company.