And, if all companies adopt this technique, that's 4+ hours
per company. If a reasonably-competent person applies for, say 100 companies and expects a 10% callback rate[^], then they're dedicating 40+ hours just to do tests and "challenges" before even talking to a person. That's a lot of time to blow for a mere chance of talking to a person. I mean, I guess I'd do it--especially if I was desperate for a job, but if I was still employed I'd think twice and maybe find a way to grin and bear my current employer a little longer...
I think when your company comes up with a hiring scheme, they need to analyze it, understand and be cool with the kinds of applicants their system is screening out. In my view, most SV tech companies' practices tend to screen out the "skilled professional who's currently semi-comfortable at her existing job" and filters in the "desperately needs a job and will jump through hoops and put up with whiteboard hazing to get one." Which class of applicant does your company want to court?
[^]: I consider myself "reasonably-competent" and the 100:10 ratio was about what my last job search was like a few years ago.