True, you could very well cannibalize your own advertising efforts. I think that may be a problem for brands who already advertise themselves. For example, I agree it makes no sense for Ebay to place an ad for the search term "ebay" if they are already the first search result. If you have a well known brand, you will have a lot of people who will search you out directly. In that case it's a matter of running campaigns and see what sticks - if you spend 10% of your budget on an agency's ad campaign, and very few new customers turn up, you may be wasting money.

But if you have a small to medium online shop, I think it makes sense to have someone run ad campaigns for you on as many domains as possible - and pay only for high quality traffic. If you re-sell shoes from Nike, you will have a very hard time getting a click from someone who would have bought at your shop anyway, because your competitors will pay for Google PLA and other ad campaigns and anyone who is looking for a good offer will run into their ads.