High PR links are *almost* NEVER harming links. They're high PR for a reason (Google sees them as authority sites), and if Google thinks the site is just a link farm, then they won't pass any link juice to other sites... simple as that.
The only problem is when the majority of your links are link farms from irrelevant sites, then that may raise a red flag in Google's eyes. I've been penalized using networks like SAPE before and those are all high PR links. Other than that, yes you're right that it's not a big deal if you get a small number of 'bad' links. I can't say there's no such thing as negative seo because I've never tried it before... I'm sure it exists but it's probably very difficult to implement.
Fair enough, thanks again for your input mate.