Have you ever clicked an ad you were not interested in because you hated the advertiser and wanted to unfairly drain the money out of his pocket through click fraud?
Remember that when someone is getting paid, it means someone is paying
, and when you arrange for them to pay without getting what they are paying for, that's a form of theft. I am pretty sure you wouldn't like it done to you if you were paying for ads.
It also leads to advertisers not wanting to pay as much per click, which means instead of putting more money in that site's pocket, ultimately, you are helping to remove it (and remove it from my pocket, as well). It is quite frustrating as a webmaster, to watch the total number of clicks double over time, while watching the total earnings fall to less than 50% of what they once were.