I actually dislike it for the reasons you stated. I don't see this negatively impacting a botnet at all while still annoying regular users with the delay. Also

I actually dislike it for the reasons you stated. I don't see this negatively impacting a botnet at all while still annoying regular users with the delay. Also, even if they are paying for it, the "proof of work" is probably less expensive as captcha breaking. What I'd really like to see is a more sites that do analysis to block bots, rather than annoying users. Spammers generally need to place a link and do so quickly. It seems much easier to target people who post links in their first 5-10 posts than to do anything else.