> Even a short phone screen where I just ask someone to code a simple question takes 20-30 minutes of my time (plus however much time I need to get back in the zone), so HackerRank is appealing if only to weed out the very worst candidates.

But you took 20-30 Minutes of their time too, the only difference is that you get a compensation at the end of theses 20-30 min, they don't. If someone told me, "OK , i m gonna waste 3 hours of your time in exchange of a mere promise of a job", i will simply decline the job.I work 9hr/day , I have better things to do with my free time...