Stephen pretty well said everything I would have said on the subject, and said it very well.
Not all backlinks are created equal, nor should they be. Generally speaking, the easier a backlink is to obtain, the less it will help in terms of the overall marketing scheme...and that's how it should be. Good sites should get organic links and traffic. Bad ones shouldn't.
There's one thing I'd like to add to this: a bad SEO backlink can be a great backlink in terms of direct traffic (something that reduces the reliance on search engines, who really don't owe anyone a thing and are capable of shaking things to the core on a dime.)
Let me give you an example of a bad SEO backlink:
http://blogs.tap.ibm.com
Go ahead, click on it: you'll get a DNS error. That means the site's either down or doesn't exist, right?
Wrong.
That site actually
does exist, but it's not accessible to the outside world...it's part of IBM's internal network. It's accessible to anyone that has access to IBM's intranet, but not to the rest of us worthless peonoids. No precious PageRank is passed, no direct SEO benefit can possibly be obtained, so most SEOs would look at this (and have, when I've asked them) as a "crappy link to get".
So how do I even know about it?
I wrote a blog post that went through a 4-hour ordeal I had with IBM outlining their crappy customer service policies pertaining to a machine that they sold a client of mine an extended warranty for, something that IBM never records by default (yeah, it's stupid, but then again...it's IBM.)
I'm not sure whether it's because I used an IBM Technorati tag or what exactly the deal was, but a UK IBMer picked up on it and apparently wrote something about it on his blog with a link to my blog (don't ask me what it was he wrote, because it was part of blogs.tap.ibm.com). I saw hundreds of visitors over about a 2-3 day period to the blog. They mostly from the UK as opposed to the North American and Indian departments I was complaining about, but they were interested visitors, and I was able to engage a few and get official confirmation about the aforementioned blog network. I also had confirmation that I had at least gotten the attention of a large number of IBMers and made them think...sometimes that's how things get started.
So remember...a bad SEO backlink doesn't always equal a bad backlink.