You know, a lot of users don't even realize its spam. That's kind of the idea. Some of them seem to enjoy it, even.
I'm relieved that the comments on this are mostly against it. I was sort of worried. A captcha is a very delicate balance between not pissing off the good users and keeping the bad users out.
And let's not forget what seems to be the most effective method of cracking a captcha, to just proxy it to an actual user who thinks they are verifying themselves for some other site (porn). This doesn't address that at all.
Email is only one of the many avenues for spam. Try running a search, using Craigslist, reading a popular blog's comment or reading Youtube comments...