It's a pretty effective technique

Depends what you're protecting. In general I've found the opposite - even ignoring the accessibility issues. For two reasons:

• These things can be read by computers if somebody clever is coding them. Noticed how much more distorted the images on Yahoo are getting? Now I'm getting them wrong more often than not and I have pretty close to perfect vision (in one eye anyway :-). Escalation can only lead to images that more and more people with "normal" sight have problems with.
• There are hacks to get around them. The fake-registration technique you mentioned is in wide use now. For anything that involves even a small dollar value for getting a registered account the Evil Person hires sweatshop labour that just sits there and registers accounts eight hours a day.

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