Why CAPTCHAs are not a complete protection against blog spamming

Because there are "bots" around that aren't actually bots but real humans. They're entering obscure search strings into msn search, come to my website, and then post their spam to my comments, and enter the code from the CAPTCHA. I would never thought this could happen, but it does happen, and it's documented in my Apache logfiles.

Nevertheless, this was the first comment spam for almost 2 weeks, so CAPTCHAs do help, also when it comes to keeping off Aspies.