Every year wikipedia is survive through public donation. If you are jimmy wales and out of fund to run wikipedia, what would you do? Closing wikipedia or selling banner ads to survive? If wikipedia have banner ads, as a reader or researcher can you accept it?
Yes I could accept banner ads on wikipedia - to pay for the web hosting alone, and possibly a *basic* salary for those organising it, which would mean very few adverts are needed. But there is a danger of profiteering once an organisation becomes commercialised. The actual work on the articles comes from volunteers.
Wikipedia do hiring staff, if not mistake they got about 70++ staff. The cost overhead to run wikipedia is high. Agree with you, once it become commercial, many thing can be happen. E.g. they could make injection of promotional content to existing article.