The only time an ad might be remotely useful to me is if it's product placement which is on topic for a search that I am performing where I am already looking to buy something. At any other time it's just wasting bandwidth (which on some networks might actually cost me).

I think that 'ads' are the wrong tool for the job. Ads are about 'making impressions', but doing so is actually an attack on the intellectual process of the subject that is being impressed. The very least an Ad should do is compensate for that attack by delivering actually useful and desired information. An ad should know from the context it's presented in what that user seeks and attempt to deliver value to that user. Ideally it should be about finding a match between consumers and the producers that match their need. The toxic part of ads is that they are not driven by this bounty, but instead by attempts at various companies to become the thing that fulfills a need which might not exist or which might be actively unhealthy.