> when you have to solve one every time you try to post

I am only familiar with Wakaba doing this. All other examples of captcha that I can think of work for sites that use accounts.

And Wakaba's captcha is intentionally very easy to read. The only people who need worry about the captcha are mass-posters like Sling, and there's a patch out there that allows them to circumvent that.

> only request one when someone conencts to your dummy site

Which is why you limit IPs. A server-based approach won't work well, containing the damage. You might get several accounts, but that's less of a problem for the target than a few thousand.

> the server fetchs a captcha when it needs one

Because one way around the limitation on IPs is to hotlink the captcha. This prevents that.

> One dummy porn site can service the spamming needs of many, many small sites

Yeah... except that people don't have much reason to do that. Why would I go through all that effort for Joe Blow's Tiny Imageboard? Making a viable porn site so you can crack captchas is a lot of work.

Again, it raises the bar. That's the entire point.