Ads hail us with who they think we are and with who they think we want to be. How an ad engages the viewer is linked to issues of ideology. Take for example Reebok's "I believe" campaign. Though this is the generic you and an inclusive I, it nonetheless invites the viewer to feel the ad speaks to them. Ideologically, this ad appeals to those who feel their lives just aren't quite right -- maybe a little depressed, maybe overly self-critical because others have laid that trip on you. There is more than a little therapeutic ethos in this ad.
Ambiguity as to the precise referent of the personal pronoun in these ads is a structuring device which assists in generating the illusion of identity (equivalence), and then individuality. Ads suggest a relationship between the desired "you" that is the model and the actual "you" that is the reader.