Onsite SEO is no-longer sufficient to get any decent long term ranking in the SERPS, which is all really a dedicated designer could do. If you want decent rankings for competitive keyword(s) then it can take a lot of time, effort and money.
A good SEO will work with your site for a longer period of time, making small adjustments to keyword density, adding relevant content etc..., utilising and implementing the latest technologies such as G Sitemaps, RSS feeds etc.... Not to mention obtaining (quality, relevant) backlinks with targeted anchor text.
Getting a site into a highly contended position is also not simply enough - for example I used to own a mortgage site, and from redesign it took me 5 weeks, full time, to get it ranked in the top 10 for my target terms (out of about 15 million results), but it was almost as much of a job to keep it there as it was to get it there in the first place - and that's if there is no change to the SE ranking algo, in which case the site may need to be re-optimised to reflect the changes of which neither the webmaster or SEO has any control.