I'm a somewhat inactive Wikipedia administrator (and long time editor), and it was paid editing that drives me up the wall. Paid editors are tireless, relentless, and never accept defeat. Several attempts to shore up the policy against paid editing have been attacked by paid editors themselves who stand in the way of every attempt to reach consensus. It is positively infuriating.
Paid editing undermines the integrity of the project and there is no question that many corporate articles suffer as a result. A now-retired admin, Yellow Monkey, did a systematic review of articles known to have been written by paid editors (but which were subsequently reviewed by other editors) and noted that they had a clear bias in favor of the person or organization doing the paying. There's absolutely no question in my mind it is a danger to the core functioning of the project.
If you engage with paid editors, you will likely spend several hours a day "discussing" with the editors. They will wear you down. If you take a break, they declare they have consensus. You will get nothing else done, nothing else written, and nothing else edited for months at a time. The fact of the matter is a volunteer editor just can not match the endurance of a paid shill.
Paid editors should be banned on site, and the articles they edit should carry a banner announcing that a paid editor was found to have been modifying them (making companies averse to hiring PR firms to do the editing).