Chris Maunder wrote:

No you don't. SEO is important if you engage in SEM, but SEM is such a small, small part of a solid online marketing strategy. In fact SEM is only a tool, one of many you should use, and so could be part of a strategy, but a strategy also includes things like understanding your audience, the market, your competitors and your product. (I add the last one because I speak to too many marketing departments that don't actually understand their product).

I tend to agree with Chris. Pay-Per-Click campaigns have their use, if you need instant traffic to your site, but it can be costly. A solid online marketing strategy should include long-term approach to SEO, including optimizing your site to target the right keywords, on-going content creation, and link building from relevant, authoritative sites. PPC gets you site visitors, but doesn't do anything for your website in the long term. A good SEO campaign will build equity to your website, and eventually bring in organic traffic so that you won't need to pay for every click.