You can't use apps, look at friends apps, or do anything else interesting.
Facebook has made itself a social address book, and dragged down everyone who bought into their "platform".
I suspect they noticed a lot of people look at a profile, say "oh look an album", start clicking through and then decide they were done and go back to their feed. This design makes it more likely you will return to where you left off (without having to push back a million times) to finish reading the profile that you were already looking at.
Yes, the advantage you point out is valid. It just comes with a whole pile of downsides and I can't believe they would do that for that one minor use case. I was with a friend the other day who was getting so irritated that when I said "just hit f5" to restore the old interface she was so happy she made me dinner in gratitude.
The only other advantage I can think of is that the previous-photo button is more mobile-friendly.
It almost feels like a political issue--with all of the longterm controversy and animosity that Facebook has stirred up amongst us, we've been quietly waiting for something like this to happen. I'm glad to see that so many of us appear to be rallying around what appears to have the potential of being a "Facebook killer".
I shouldn't be so emotionally invested, but after years of Facebook acting like assholes to all of us, I hope Google lays complete and utter waste to them. Facebook has under no uncertain terms been trying to swallow up the web, and before seeing this level of competition I thought they stood a chance of accomplishing that goal. Perhaps not anymore.
I deleted my Facebook account a long time ago, and I honestly am not interested in social networking after that experience. I'm not sure whether I want to do the Google+ thing long term, but I'm definitely going to join if for no other reason than to get all of my friends to. Maybe I can get some of them to migrate completely. I trust Google a hell of a lot more than Facebook. (Perhaps I shouldn't, but I do.)
I wonder what this holds for their upcoming IPO? People are fickle and can ditch a service rather quickly (MySpace, hotmail/yahoo->gmail). Even with the wealth of photos and memories Facebook has accumulated for many users, I don't think it will be able to retain them by touting that data alone.
This is going to be very exciting to watch. What a good show you've put on for us, Google!