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I think its worth the try, at least for a while:
1. For statistics (how else to really know how few people actually click on results (even no1) when searching for that keyword. I would guess Google would not like to publish that type of statistics in general, but I would not be suprised if only 10-15 % of searchers actually click on the number 1 result, or for that matter, on any listed pages of the SERP.
2. You get the odd (yellow-page addict) searcher only clicking the add.
3. You can be number one for certain keywords in normal Google, but not necessarily if the keyword or keyphrase is in the middle or in parts of a search querry.
4. You can lead the searcher to a third page on your site via the advert. (supposing you are already first and second, as most excellent SEO'ers here are ;)).
My guess ist that top 3 results have a Click rate of 5 to 15%, depending on how their titles and descriptions match with the search terms.
I can confirm that many users click the adwords even when the site is present in the regular results... depending how appealing the description is in the context...