"suppose a prospective client or employer saw your face on an advertisement for some banned substance"

But this is not what Google does, or am I missing something? AFAIK, what they actually do is more akin to shopping basket analysis supermarket chains do. So my question remains: How exactly does it harm you? Has Google ever linked you in a publicly identifiable way to some product? Moreover, if you feel bothered by the way they handle things, why don't you just stop using them?