[randometc]

Assuming your whiteboard session is a good test of basic CS proficiency, the real question is "does basic CS proficiency correlate with effectiveness at the job?" - if you're like Google the answer is probably yes, and if so you only have to have tolerance for false negatives. Given enough applicants that's clearly less of a problem than false positives.

My other point is that some companies might find there are other qualities they're looking for that occur in the pool of people they pass on. That's what's really worth looking at, I think - all the while being careful not to mistake "good cultural fit" for "promotes long term monoculture".