Yeah, but we know that Kirk and Spock etc are running around doing just fine in the Prime Universe, so I give as much of a shit about these new guys as I do that Riker who got blown up in that Borg war universe in Parallels
We met Shabby Riker about one minute before his Enterprise blew up. We have seen a 2 hour film of these new guys, and will get another 2 hour film to get better acquanited with them.
Using your logic, we can't care about any original movie character
(any original character in any original movie) simply because that the character is not someone we already know or have cared about before the movie. I have seen plenty of films that featured original characters whom I grew to care about during the film.
I think Abrams and company did a decent job making me care about most of the characters in the first film -- although being an origin film, the creative didn't have a lot of time to do so. I think I will care even more about them in this second film as I learn more about them and watch them interact.