This already happens and inevitably will become more common as Wikipedia's profile rises. WP might as well get out in front of it with policies that make it easier to police and verify.
Currently, PR firms who are hired by companies to raise their profile already add biased, poorly sourced puff pieces to Wikipedia. They are promptly shredded by the community and deleted in nine cases out of ten. They do, however, create a lot of work for Wikipedian volunteers, usually because the PR people in question know websites generally, but nothing about the rules and culture that govern Wikipedia. They also do not generally disclose up front that they have a business relationship with the company they're writing the article about.
There's an argument to be made that there's an advantage to replacing these PR firms with people who are already clued in to Wikipedia's culture and guidelines. They could communicate up front to a client what will and won't fly on WP, and the best way to add verifiable information about the company without running afoul of neutrality and verifiability guidelines. If all these paid editors do on behalf of their employer is add content and provide sources, as long as their work is in accord with policy I don't see a reason to care that they are getting paid.
There are freelance wackos and fanboys that attempt to sabotage or whitewash pages about companies and other institutions as it is. How are paid editors different? At least you could require them to declare their influences. Make stringent requirements about disclosure, and allow paid editors to edit and provide info in talk pages, but not to take any administrative actions on the pages they're paid to edit. Any violation results in a topic ban for that account.