The thing with captchas is that they're basically security by obscurity; the less sites/services use one, the less spam will get through. That's because if you'

The thing with captchas is that they're basically security by obscurity; the less sites/services use one, the less spam will get through. That's because if you're someone like Google or Facebook, the payoff for writing bots specifically to crack their anti spam measures is huge, while the same isn't true of the average blog/forum/wiki. So a homemade solution like the one in your post works fine for many sites. The bots written for forms without captchas can't solve it, and their developers won't waste the time changing the script to fix that, because it's not worth the effort. If your site/service becomes extremely popular on the other hand... you'll need a more robust anti spam solution. And given how thoroughly things like Recaptcha have been cracked, those won't cut it there either.