I never used to. But I recently watched a bio on the 2 Google guys on the History Channel. It talked about how they went out of their way to make sure the integrity of the priority order of the Google searches would stay uncompromised - meaning they wouldn't change the order of the results because of
paid ad sponsorship.
Their company mission statement has always been, "Do no Evil."

So to guarantee this, they
purposely put sponsored ads in a well separated box or off to the side so no one will confuse them with the regular searches.
The Google AdWords sponsors help PAY for all of our free Google searches, the GMail, the Google Map, etc. to stay in business.
Google receive 3¢ for each time someone clicks on one of their ads.
Since I must do like 10-25 Google searches a
day, if I see a link that shows up in the regular results, and I want to look at that site, I'll purposely click on the AdWords link instead.

It's the least I can do to keep them running.