From how I subjectively see it, 5% of effort gets rid of 90% of spam. Of course, this would never catch any targeted attacks, but in the grand scheme of things,

From how I subjectively see it, 5% of effort gets rid of 90% of spam. Of course, this would never catch any targeted attacks, but in the grand scheme of things, those are exceedingly rare. In other words, good luck keeping out the remaining 10% without significantly degrading the experience for the innocent majority of your users.