Exactly. No public posts? No public profile. Mention to the user that they're doing something "profile-oriented" that's going to start appearing on their profile if you really want to do the silly "activity-log" approach to a home-page (that is, every YouTube video or Blogger post the user likes and comments appears on their page) - once the user starts doing actions in public, that's the time to lazily create the public page, and that's the time to ask the user the hard question of "hey, I'm going to start collecting all your public commentary and public likes into a single place so everybody can see the stuff you say and like, is this good? yes? Awesome. No? Let me set up a pseudonym for you to do that"

Google obviously wanted people who were active public commenters/contributors on their properties (YouTube, Picasa, and Blogger) to automagically be part of Plus, and that's not a terrible idea (obviously treating commenting and sharing as the exact same operation without distinguishing it to the reader is a terrible idea, but that's just a detail). But why foist Plus on all the lurkers? Lurkers don't add value to Plus anyways.