A properly run PPC campaign is beneficial even if you already have an organic #1 Google ranking. High rankings do not guarantee high click-throughs. It is therefore better to cover all your bases.

A thread on HighRankings.com has asked is it worth it to keep your PPC campaign going if you already rank #1 on the major search engines without it, and industries leaders have weighed in favor of PPC. Kevin Gibbons, a Google Adwords prfessional at SEOptimise writes:

In my opinion if you are bidding on a keyword which has a good conversion rate you should try and maximise the amount of traffic from that keyword, if you are also ranking well organically that's great news for site but why give up the ad spot and additional quality traffic to another competitor?

Indeed, covering your bases seems to be the consensus from the industry professionals. Phoenix over at SEORoundtable says:

High rankings in those position will not always warrant high click throughs. Its best to cover your bases. [...] the PPC ad itself can be more successful than the organic listing. Whatever the case, if you are a small business or in a very niche area, branding can be very impactful to your customers and users to see your dominate the area for your search terms. This adds credibility and trust to name and product which can lead to bigger profits or better business oppourtunities.

At the end of the day, marketing and advertising are about exposure, and being at the top is no guarantee that you'll stay there. Indeed, the biggest brands, such as McDonald's and Coca-Cola, seem to have the largest advertising campaigns. In the world of e-commerce, then, it is good business sense to maintain your PPC campaign even if you are already #1 on Google, Yahoo, and/or MSN.