Jwray: Ad comments will benefit all users and honest companies, at the expense of dishonest companies.

In theory yes. I think the problem is that the people paying for the advertising are often agencies and not the companies themselves. Normally agencies do not want to see negative feedback because they are usually being paid to do promotional work and the negative comments would (perhaps unfairly) reflect badly on them. Because of this I think most big brand agencies are going to be nervous about allowing feedback.

The agencies often know a product, advertising campaign or promotional video sucks before it goes out but they are still paid to promote it.