I think that you're fooling yourself if you don't think that G+ is going to have Farmville on it soon. It's too big of a draw and too big of a money maker for it to not have them.
If there's some g+-only games, I'd see that happening.
Google doesn't need to enable that, because they don't really have to make money by taking a cut from the game developers. So I hope they are much more strict with what developers can enable for users to share.
However, if it shows up like on Facebook, I (and others) will likely leave G+ for that reason alone.
Google shouldn't try to "beat" Facebook (and I don't think they are trying to). Differentiation is key, since Google really makes it's money elsewhere.