So, for what demographics does this publicity stunt improve the perceived legitimacy of Google+? I can't think of any negatives.
Google+ has already caught on in the techy/engineery crowd (still not to the extent of Facebook, of course, but Google can probably afford to let this grow slowly), but while now Twitter and Facebook are household terms, "Google Plus" probably is still unknown to many regular folks. These kinds of publicity stunts are entirely aimed at changing that.
I think it's a smart move, and honestly, it's nice to see someone promote their service by holding chats with the POTUS rather than a Kardashian or something.