no i dont. the content of a site that links to you is only used to determine whether or not the link is in context to what is being linked to -- in other words.
You have site about birds. Do other sites about birds link to you?
Yes: Google likes that, and PR might be positively affected
No: Google doesnt care, and PR probably wont be positively affected BUT WONT BE negatively affected.
Do you understand the difference?
Google doesnt punish sites for who links to them.
Youre also confusing a backlink with a comment within a link, and thats NOT the same either.
A backlink is someone that Links to you.
A link left on your site via a spam trackback or comment is a link from you to them -- and thats a backlink for them, NOT you.
Does Google punish sites that link to known shit sites? I have no clue, not do I care to. I take care of my spam, and always have. Thats part of being a responsible web master, regardless of the web application I use.