Stoney DeGeyter has written over at Search Engine Guide that not every client needs SEO. Sometimes the usability needs to be tackled first. Using an analogy, he explains that your website is like a restaurant. If the restaurant isn't accessible, people might not hang around. He drives the point home with:
By fixing usability issues you can sell more while without having to spend a lot of money always trying to bring in a lot of new traffic. A little goes a long way. Once you are able to increase your conversion rates, every dollar you spend on marketing and SEO will be more effective and have a much bigger impact on your profit margins.
This is important because not everyone is ready to embrace a search engine marketing campaign if they're not able to make their website more usable. This is true for PPC as well, as echoed on Sphinn:
Excellent point! I see the same problem (maybe even more of a problem) with those looking to do PPC. Companies want to advertise via Adwords but have a site that clearly will not work in paid search.
So here's the suggestion as proposed by Search Engine Guide: make a usable website first, and then focus on search marketing.
Forum discussion continues at Sphinn.