Of course you have to have a mechanism. Running an online business of any kind, but most especially with user-generated content, requires an unending investment in forensics. It's no different for Wikipedia than it is for eBay or even Hacker News.

Most of these companies would cease and desist after one politely worded but firm letter.

If they were the victim in the case, they would probably provide money and/or resources to track down a rogue PR firm, an unethical competitor, disgruntled ex-employee, or miscellaneous vigilante whack-jobber.

I don't think we need to get hysterical like some responders and assume that every corporation is out to do black bag jobs on each other. If they can spend a few bucks and get some easy astro-turfing, they will. If the penalties are such that it's not worth the bother, they won't.