It's a good point that some spam "bots" could be human. I was running a free email site when we saw a lot of strange account creation. We had recaptcha enabled and knew it hand't been cracked.

It turned out (based on IP address analysis) that the accounts were being created by humans in the Philippines and then handed over to spammers in Dubai. Ah globalization...

If you have an efficient spam vector, it's not unusual to see low-wage humans manipulating the system to get around captcha.