Contextual ads, such as the ones served by Google's Adsense, can produce a few extra bucks per month, or even cover a couple of bills if your site has a lot of traffic. However, if your website business model is based on offering your company's products and services, then you are probably better off without contextual ads.
Imagine this scenario: your website is an online outlet for your small law firm. In your site there is a page about your services for divorce matters targeting the keywords “divorce lawyer”. As your page is trying to promote your services in this field, the Google Ads on the sidebar or around the content will display these ads:
See the problem? The goal of your website is to promote the products and services of your company, while contextual ads will recognize the theme of your pages and display the ads of your competitors for whatever subject you talk about on each page. So no matter how you plan it, if you use contextual ads more often than not you will be offering your competitors' services along with your own.
Although every time a visitor clicks on the ads you will get a small monetary compensation, you are also asking your potential clients to leave your website and go checkout your competitors' instead. At the end of the day, whatever amount of extra cash you made through the ads on your site probably won't outweigh the business you may have lost.