The problem is that for captcha to work properly, there has to be an enormous number of possible captcha words and phrases.
If people use advertising slogans for captcha text, then the advertising execs will only be able to come up with a very limited number of slogans (they are, after all, only human and from my experience they are not the type of humans that work too hard). And if there is a limited number of captcha texts, the hackers will easily program bots to recognise them.
You could also have your ad contain a regular captcha image.
The area around your captcha will be getting eyeballs, it's not a bad idea to use that space to pass a message to your users. Ad blindness is always an issue but that's why changing the way you present your promotional content, every now and then, is a good idea.
If people use advertising slogans for captcha text, then the advertising execs will only be able to come up with a very limited number of slogans (they are, after all, only human and from my experience they are not the type of humans that work too hard). And if there is a limited number of captcha texts, the hackers will easily program bots to recognise them.