As an SEO expert, I do site evaluations daily. It amazes me how people are willing to put Google ads all over their pages in the interest of earning a few extra bucks without considering the consequences to their business. Think about it. If you are selling a service or product, why would you allow the ads of your competitors on your site? Are you willing to sacrifice a sale for the few pennies you'll make if someone clicks on the ads? It's ok to include Google ads in some instances, even I have some ads on my site. Knowing where and when to allow google ads on your pages is key to maximizing revenue from your traffic while minimizing the chances of lost business.
Google ads are based on the content that is on your site. Because your content is about a particular product or service, the ads on your site will also be about similar products or services. If the ads are compelling enough, people will click them and leave your site. Sure, you'll make some chump change, but how much would you have made on a sale if you hadn't given your customers a window to the competition?