There has been a lot of back and forth on this issue but I'm not sure we have properly explained PPC to you. When you run a PPC campaign -- and I will use AdWords rather than Overture as an example -- you get your ad displayed on the page with the SERP when a particular keyword or keyword phrase that you have bid on is used by a searcher. As buyer of PPC ads you only pay when someone actually clicks through to your web site.
If he were to not have me or anyone else optimize his site and choose to go strictly with a PPC campaign with Overture, when he completes his campaign (say, in a year of so) will his site be any better off with the SEs then than he is now? In other words, does he have to keep up the PPC campaign going indefinitely to stay visible to the SEs and will his site becomes virtually invisible again to SEs once he pulls the plug on Overture in the future?
I want to strongly convey to him the importance of SEO and how it should be used in conjunction with a PPC campaign, not run a PPC campaign INSTEAD of doing SEO work.
Now the minute you end the campaign and withdraw your offer to pay for the click through the seller -- Google or Overture etc. -- will immediately stop showing your ad. Why would they continue with a paid advertising program if you as the buyer are no longer going to pay?
So the only possible ongoing benefit from a PPC campaign is that people who have found your site bookmark it and return via their bookmarks or favorites and make repeated purchases. In other words if your product is not a onetime purchase type of item you may build a repeat business, customer base by using a PPC campaign.
If you do good SEO work them you may come up at or near the top of the SERP when the keyword phrase is searched on. This will cost you nothing except the fee for the SEO work.
However there is really no good argument for why these two thing must be done together. If the ROI is positive on the PPC campaign someone might well decide to never bother competing for a high position in the index. Remember that a high position in the index can disappear without notice and without apparent explanation. Just read some of the current horror stories on this forum.
But with a PPC campaign, once you get your ad copy tweaked and working you are almost assured of a continued flow of visitors. And you know that your ad will show up every time a specific keyword phrase is searched on.
Now what happens in practice is that some companies will use a PPC campaign until they have achieved a good place in the index. Then they will reduce their advertising expense and live on the free listing. Other will continue to do both -- there are a lot of people who don't like to click on paid advertisement -- and by being high in the ranking as well as buying PPC you will get clicks from both campaigns and increase your traffic over all.
I hope this explains PPC a little more fully, but despite what Qwerty has said I think most of us agree that you can run a perfectly successful PPC campaign without ever worrying about SEO. What you do have to worry about is user friendliness and converting visitors -- who are now paid visitors -- to sales.