The point is, a company can try to artificially shape public opinion by placing an advert. Without trying to be moralistic (or take any view on its relative me

The point is, a company can try to artificially shape public opinion by placing an advert. Without trying to be moralistic (or take any view on its relative merit) advertising _is_ largely an art of manipulation. In the context of Wikipedia - advertising would destroy any value Wikipedia has, because it would mean that companies with commercial interests would have a way to counter (and artificially influence) opinion. It would also create a situation where companies with the most money, would have the most clout. I think it would amount to a disaster if advertising was allowed in.