I doubt there are many OCR reader bots which are actually reading Captcha graphics successfully; they're probably making inroads some other way.
There are plenty, actually, but forum admins hit a tipping point of diminishing returns really fast, when they start making their Captcha harder. That is, they can start blocking out the bulk of valid registration attempts along with the flood of spam (which ebbs and flows).
A mild Captcha is fine, in my opinion. If we only throw out MOST of the bathwater, so what; we can always towel-dry the baby by hand.