Assuming for a moment that this scheme does exactly what Google claim it still won't measure the actual effect of advertising. In order to do that they'd also need to measure the negative effects - who bought what instead of a product that had been advertised at them because the advertising pissed them off.
Nobody in advertising or marketing will risk doing that. As long as they don't know they can plausibly deny that such an effect exists. The advertising industry can keep on selling to marketing and marketing can keep drawing their pay. Knowledge would seriously disrupt this cozy arrangement.
Remember folks, the only thing the advertising industry sells is adverts.