The biggest problem with online advertising is not tracking users. It's a lack of trust between advertisers and pretty much everyone else. If you're going to pay for an ad, you want to be sure it was seen by a real person. I'm not sure that's the concern any more because click-through is more important than "seeing" an ad. Regardless, the goals are to make sure it's easy for a given advertiser to get on many web sites, easy for a site to get ads, and also possible to prevent fraud since there will obviously be multiple parties involved.

I suspect tracking users was an offshoot of just verifying that users were real to prevent fraud in the ad world. Not saying any of it is OK, but it seems like the way to prevent tracking is to find a way to verify authenticity while also preserving privacy.