The logical side of me asks:
"How could this be? How did all of these Mirror Universe people all come together to be on the Enterprise -- AND Mirror Kirk, McCoy, Scotty, and Uhura just so happen to be beaming up from the Halken's planet AT THE EXACT MOMENT our heroes were?"
Although I think the idea was to show that if there are infinite universe, there are infinite possibilities, so they get some slack. It only starts to get weird when they revisited (and kept revisiting that universe) and mirror counterparts exist for every character 100 years later.
STXI was a little bit different though in that it wasn't addressing infinite possibilities, just one. That changes expectations a little.
Why would they have Vulcan destroyed just so that Old Spock can go do some "simple" calculations and bring it back into existence? It would be a cop-out, just like every single other tired reset button in Trek.
I think mostly because it conflicts with WWSD (What would Spock do?
) in the show. This is just one of those moments that people need to realize that this is a reboot (or that it's just fiction). These characters behave differently than their counterparts that we saw decades ago so it can't really be treated as some sort of strict continuity.