Filtering PR from adult to mainstream
 

Are there any penalties of hard linking mainstream sites from porn sites? I know it doesn't make sense to get traffic but does it matter for backlinks?

PR has nothing to do with relevancy of the content and the type of site, Raw PR just flows from page to page irrespective of the content and type of site. A PR7 link from adult site alone can make a new mainstream domain PR6.

And Yes, of course it will give you backlink but may be it doesn't have that much weight which a relevant link could have, but still it does. (by 'weight' here, I don't mean PR)

And who said you that it doesn't make sense to get traffic? you think anybody looking for porn doesn't browse mainstream sites? Its just the matter of interest. May be your visitor also have interest in heavy bikes or itunes, isn't it?
Have not you seen Image and File hosting, poker, myspace type of ads served on adult sites via AdBrite?