I love this approach. I am good at what I do, and appreciate the opportunity to prove it directly. Spending a couple hours solving a technical problem or writing an essay is much easier for me, not to mention sending a more reliable signal, than trying to figure out what networks / buzzwords / whatnot various people in an opaque organization are going to want to see. Not to mention, it communicates to me that the people doing the hiring know what they want and how to get it, which is encouraging.

I believe there is a happy synergy here, by the way. A candidate who welcomes this type of test -- who sees a direct work sample as the easy and efficient way to prove themselves to you -- is probably someone who knows their work is good.