Alessio,
It's a valid argument, but one we aren't going overboard to prevent right now. Yes, it's pretty easy to get a Google account. However, they do have mechanisms to report bad accounts. In the month we've been using G+ on this site, and the few weeks of reports from users, spam hasn't been an issue yet. That's certainly not a long-term trial nor exhaustive, but it says that it's not an immediate problem.
As we add more authentication providers (Twitter coming in about a month and LinkedIn late this year), I can see having individual options for activation per provider instead of the one setting we have right now. It still won't be perfect, and there are even spammy Facebook accounts, though rare.
There are also plugins for detection of spammy IP addresses and/or email addresses. We use SpamBotCheck on this site for normal Joomla users, but it doesn't check for users registering through a social network using JFBConnect. If it spam becomes an issue, we'll definitely add more features to JFBConnect to allow better verification of certain networks (so that G+ users may have to go through email activation while FB users wouldn't). Additionally, I'd gladly reach out to the SpamBotCheck developers and implement support for JFBConnect and recommend that, as it's a great plugin. However, for now, it just hasn't been an issue.
Obviously, if you're seeing 'bad' behavior on your site, or have any suggestions or feedback, we'd love to hear it. Facebook has been very good about policing their accounts (though it's estimated that at least 10% are fake accounts), which has made things easy. As we expand into new social networks, we'll have to re-think some general assumptions we may have, and your feedback, like above, is invaluable in this process.
Thanks,
Alex
A personal consideration about G+ Login
