It is much easier to send targeted traffic to an adsense page and earn money than to fraud adsense.
Moreover, click fraud is really bad, since you steal from poor publishers. How many times have you heard stories of people losing a lot of money with adwords to send traffic to their affiliate page and had no conversion? Well, probably a good part of guilt is taken by the fraud clickers.
I would never do such a thing as click fraud, and this not because i am afraid of getting banned as because i have some ethics in my business.
And last but not least, google is not invincible. There is a guy i know who has developed a script that fraud adsense and he has been paid and not banned for a while now, despite the fact that he sent to them only fraudulent clicks. Before he developed this script he has had 27 adsense accounts banned. I know they were 27 because i created them for him.
The huge drawback is that he has to earn within certain low limits otherwise he will get immediately busted for poor conversions.
But, anyway this was just a mention for those guy who used to say that google is invincible- it is not, every program and any security can be hacked if someone is really dedicated.
But, is it worth the effort? I earn more from adsense legitimately than he earns with his script.