Yes, being paid alters judgement. So does joining Scientology, believing in Astrology, being against gun control, being for gun control, or any number of other reasons for advocacy editing. I could make a case for Wikipedia being harmed more by religious or political advocacy editing than by paid advocacy editing (but see below for why I think this might change).
There are however, a couple of things that are special about paid advocacy editing which IMO require extra care.
First, Scientology cannot do anything to suddenly have ten times more followers. They are already doing the best they can and the pool of potential converts is resistant.
Paid advocacy editing, on the other hand, can easily gain as many paid advocacy editors as desired by simply paying more. Think of the difference between getting emails advocating politics or religion vs. emails about online pharmacies. The first is annoying, but self limiting; the second gets bigger and bigger as long as it remains profitable. Our article on Email spam gives numbers like 200 billion spam messages sent per day and 97% of all emails being unwanted. I think Jimbo is right to be especially concerned over something that could grow exponentially.
Second, we really cannot do anything to convince most religion or politics advocates to change their ways other than blocking or banning them, but we can make big changes in how paid advocacy editors operate. If it becomes well known that the advocacy editors who don't edit articles directly but instead make talk page requests get better results and that the sneaky kind of paid advocacy editor usually gets reverted, the customers will naturally go with the service that gets the best results. We can make that happen, but again it takes special care.