What exactly is "link building"? That itself sounds pretty scumbaggy to me...
In an ideal world (white hat) SEO should include things that make the site better for their users and search engines, and hopefully improve search rankings organically.
"Building" links in order to increase PageRank sounds like it's manipulating the system.
Legitimate link building is basically press and blogs relations (to maybe get links from journalists and bloggers, give them free stuff) and content marketing (invite a blogger to write for your blog, or write content on other blogs, and make links in both directions). It's not really manipulating the system, more like playing with a full understanding of the rules.
Spamming every website out there with useless comments containing your URL is more shady business and can result in a ban.
From what I understand, link building, in the semi-legitimate form at least, is contacting bloggers and journalists and saying "Hey, you might really like product X, here's a 30 day trial", sending out press releases, making sure you're in the legitimate directories like yell.com in the UK, etc.
The illegitimate end of is is using hacked forums, wordpress blogs, etc, to post links with exactly the keywords you want to rank for.
And from experience, I can tell you that most "link building" is the spammy type. On a good day, it'll merely be sending out a ton of "link exchange" request emails (a practice which Google also frowns upon, btw).
In an ideal world (white hat) SEO should include things that make the site better for their users and search engines, and hopefully improve search rankings organically.
"Building" links in order to increase PageRank sounds like it's manipulating the system.