Me and you have similar sites.
You do not add +1. I do.
You can bet that G will rank me higher (I might get a +1 one day and you get none).
I doubt they'll announce what advantage it has or how much but they'll it a social signal or whatever..."one of the many signals we use to promote higher quality sites...blah blah...we're math geniuses...trust us...we come to work on a t-shirt....change the world...blah blah". Eventually people will get the message.
Plus fear of losing rank and their livelihood will cause many to add it, just in case. It's not like you know what caused your ranking to go up, down or to 0.