PPC for keywords your website is already visible organically

The question "Should I spend money on CPC/PPC Ads for keywords I am already ranked higher through search engine optimization (SEO)?" Well the article I read today has the same answer.i.e. "It depends". I am confused and therefore sharing here to see if someone have a better idea or experience.

Let me rephrase my question by using Amazon example I just ran on my brohttp://www.blogger.com/img/blank.gifwser. Look at the highlighted sections, Ads verses Organic results.




Now that you see this visually, I would like to know why Amazon invested in PPC for keywords like "Camera", "Best Camera" when they are organically listed on Google first page at #7/8.

It looks clear to me that they have a set budget for PPC that they need to spend in any case. Also it does look like that it is working for them quite well. So should I assume that SEO+SEM should go parallel to cover each other and there is no hard and fast rule where the searchers will clicking.

This article might be interesting for some of you looking for answers.