I've never caught anyone for fraud when I've been on the affiliate end of things (just when I've been on the advertiser end), but if there's something egregiously fraudulent, like someone selling an email submit submitter software, I'd be inclined to let a network know so that they can block the fraud. The more fraud that comes in on an offer that I'm running, the more of a chance that I won't have that offer for long since the advertiser will see low quality transactions and kill the campaign. I haven't really come across any as an affiliate, though.