I think the problem with this approach is you're actually selecting for a related skill that might have little to do with whether the candidate will be successf

I think the problem with this approach is you're actually selecting for a related skill that might have little to do with whether the candidate will be successful. Some people are very good at doing the work but don't perform well under "social" pressure, while others may be very good at dissecting a small thing in the interview context and yet be unable to follow through on a large, complex task. The company I work for solves that issue by combining front-end interviews with a homework project of reasonable scope, followed by a paid work-along day. It takes more time and investment than running through some coding exercises on the phone, but results in a more cohesive team because we know before we hire not only whether someone can do the work, but also whether they can do the work with us.