The “why get an account to the site” question is going to be replaced with the desire to be rewarded for the use of OpenID. Visit site X with an OpenID provided by below-the-line marketing client Y and get Z back as a voucher delivered to your phone as a QR code. OR perhaps turn it another way round, brands are going to sponsor OpenID providers log-in real estate, to redeem the voucher offered on your log-in page, you just follow the affiliate link. Brand awareness no longer a finely honed mystery, but actually a measurable science.