[davidp]

You might be surprised at how many people show up to a developer interview and are unable to have an intelligent conversation about the problem, let alone solve it. This is a weedout question, that's all.

Bear in mind that the objective isn't to weed out people who can't solve it. There are plenty of people who are fine coders who wouldn't quickly hit the solution for any number of reasons, including nervousness. The objective is to weed out the charlatans and amateurs who managed to get past the HR resume filter, so that as little time as possible is wasted.