Depends on the niche, really. I despise ads, but I can also admit to having learned about products from them that I have subsequently purchased and been pleased with.

Sometimes the ad lets me know about an entire type of product that I didn't know existed but found very useful, and I probably didn't even by the actual brand that was advertised.

If you consider the general concept of "letting people know what products are available for purchase", I think it's hard to disagree that it's a reasonable thing to do. That doesn't excuse the manner in which it is done today, of course, but that core functionality is not fundamentally evil.

How does the first person find the product to recommend it, though? There has to be SOMEONE who tries the product without being recommended by a previous customer.