Isn't this was Google+ "Circles" was supposed to solve? It seems even if you did have just one permanent online identity, you could choose who would see what. Which it seems would work for the author. On the other hand, for public forums and anything else that would come up in a Google search would be tied to your name as well (which I think is what Moot was getting at). It seems in the end, most people would avoid being their "true" selves, even if they're passionate about the topic of a site or forum.
I'm one who likes to remain fairly private, and wouldn't want many of my Facebook friends to even know about a blog I may have, the type of Youtube videos I watch, or know of my Reddit account. Maybe I'm just old fashioned or self conscious, but I try to limit what people know about me.