Consider the following scenario: you are pitching SEO/Website Optimisation to a prospective client and you explain to them the importance of great copy and content, how acquiring links (ethically) can increase page rank, why the quality of the HTML build matters (H1, H2 tags, w3c validation etc), why keyword research is beneficial, you may drop in a few Google Webmaster Guideline or Matt Cutts references to back up your claims and rubbish the "back hat" approach as being no longer effective for good measure. Your advice is ethical and in the eyes of best practices, spot on.
Then, the client points out to you some of their long established competitors on Google and you see these competitor websites ranking in the top spots (1 to 3) for medium to highly competitive search phrases that your client wants to compete for. These websites totally contradict your ethical approach and pretty much violate every best practice previously noted. They even out perform other "white hat" competitors who are in accordance with the above guidelines.
I experienced this today. One of these well ranking websites had:
About six microsites with more or less the same copy and a slightly varied layout
Little or not textual content
I would almost say duplicate content across the sites, but there was so little of it it could barely qualify for being duplicate
All the content in Flash (with a music track that kicked in on each page load, not so much of an SEO issue - but it helps paint the picture)
Keyword stuffing behind the Flash file with a bunch of black text on black background in the style of keyword 1 keyword 2,keyword1,keyword 2,keyword 2 keyword 3 and so on...
The exact keyword stuffed combination present on every page of the website
A bunch of clearly self made links from poor quality forums and directories with little or no Page Rank
Links exchanged across the microsites
How do you explain your way out of this when this hard evidence is sat in front of you undermining your great pitch?