Look at it this way... as an advertiser myself if a campaign isn't profitable for me I stop running it after spending $500-$1k. If the majority of those clicks were fraudulent you are cheating yourself and everyone else in the Adsense program. Why? Because if the fraudulent clicks hadn't been there the campaign might've been profitable and I would've kept spending money and overall there would be more money on the table for Adsense publishers.
Of course having said that Adwords is pretty good on sorting click quality issues out. But there are some other networks out there that wouldn't surprise me if over 50% of the traffic was fraudulent.
Programmer, web developer and entrepreneur.