Ten Reasons, why I think Businesses can avoid doing SEO.

1. The results are not predictable.
2. You cannot estimate ROI on SEO expenditure.
3. It is beyond your control what text appears in results.
4. SEO takes 3 to 4 months to see some organic traffic.
5. People coming from organic traffic most of the time are not your prime prospect.  They come and go in a flash, cause they have other results to be evaluated.
6. Selling cycle is long, before the prospect becomes your customer. One will search and search many times before finally deciding on something.
7. The Search Results keep changing, as Search Engines update their algorithm. Or the competition has increased its   activity of buying links or creating buzz.
8. It is considered black hat SEO , if you divert your users from organic search hits to a landing page you want them. Search Engines may ban if they find you doing this.
9. SEO is expensive. Consider the consultant fees, the optimization fees, and links you may purchase, estimate time and effort, it will add up-to few thousands of dollars. And at the end of the day, it again comes back to conversions. Are you able to convert your traffic?
10. SEO kills the creative design part of a website. SEOs want more content. But if you want to impress the visitors in seconds, you need eye-catching design or flash.

My conclusion is that SEO is for freelancers , or self made SEOs, and Made for AdSense sites or companies in the content business which rely on advertisement for revenues. Most of them are not concerned if the visitor is interested in their content or not. All they want is to attract visitors , make them click the ads.

On the other hand,  businesses which have real products to sell, cannot rely on SEO for long term business profits. Business should consider SEM, which allows more freedom for design, the results are predictable and ROI can be calculated to the last penny. If you are not selling with SEM, then probably you will not sell with SEO too.

Few Links for more reading.
SEO - Search Engine Optimization 
SEM - Search Engine Marketing 
ROI - Return on Investment
Article on Search Engine Watch Predicting ROI on SEO Efforts