I guess it's up to delayed toolbar PR update - the actual PR may be already 0, while toolbar keeps showing 7. Am I right?
Breaking the guidelines means sooner or later the site shall be banned. Until it's not so much important, it can survive unnoticed, but algo keep going smarter and also someone can finally notice the site breaks the rules.
At the same time, there are ways to make good ranking within the guidelines, and they are the only solution for long-term web projects.
And considering how much importance Google gives to the age of your site (sandbox, if exists, and many more sophisticated possibilities described in the patent we discussed a month ago), using throw-away domains means losing most of site's potential, not mentioning the ethical aspect.