I can't understand this method.
If I want to guess someone's password, I will hack a web browser code, automatically fill-in username and password and submit. Why can this method stop it?
The problem with spammers and crackers are not people like you, but rather the ones ho hammer sites using thousands of bots running behind proxies, which send raw POST data to the action url of the site's form. Even with CAPTCHA the bots are able to successfully send the POST data using OCR, which recognization rate varies from all around 10% to high as 80% depending on the CAPTCHA system used.
For this reason, even if your method would work it is lesser bad for the website owner.
Every solution has its weaknesses, but it is rather subjective which has the least of them.