Maybe, but as things are now, you only need to increase the amount effort it takes to send spam through your web/account registration form even a bit for it to be unprofitable for the spammer and make them try somewhere else.

If all websites worldwide were to one day have captcha on every webform that could be used for spam, then spammers might start to have an incentive to try and beat it, but right now it's easier and cheaper to just keep trying sites until you find one that is not secured, then send your spam through that until the site owner discovers it and stops you.