See, this is kind of a giant problem when it is weighted against major time travel stories in the Star Trek universe. Time travel in Star Trek is not nearly as 'unpredictable' as other posters here have said (at least not while Roddenberry was around; someone started playing a little fast and loose once the old boy wasn't around to flick them on the ear).
Consider the following:
The City on the Edge of Forever, all involved parties - Kirk, Spock, and the Guardian - treat time travel as if it threatened to undo their past and alter their present.
"Captain's log, no stardate. For us, time does not exist. McCoy, back
somewhere in the past, has effected a change in the course of time.
All Earth history has been changed. There is no starship Enterprise.
We have only one chance. We have asked the Guardian to show us Earth's
history again: Spock and I will go back into time ourselves, and
attempt to set right whatever it was that McCoy changed."
resumed its shape. All is as it was before."
- Guardian of Forever, after Kirk, Spock and McCoy return
Much later, during the events of
Star Trek IV: The Voyage Home, Spock himself suggests a solution to the probe's signal by way of traveling through time.
Spock: "There is one possibility, but I cannot guarantee success. We
could attempt to find some Humpback Whales."
McCoy: "You just said there aren't any, except on Earth of the past."
Spock: "Yes doctor, that is exactly what I said."
McCoy: "Well, in that case... Now wait just a damn minute."
Kirk: "Spock... start your computations for time warp."
This idea of time travel representing a literal historical change to the same Universe the characters exist in before and after the time travel event is maintained moving forward - TNG never mentions time travel as anything but a tool for altering and/or restoring the state of previous events.
During the events of
Yesterday's Enterprise, Data makes the following assertion:
Natasha Yar: I've been working with one of the officers on the
Enterprise-C. He's, he's nice, I, I like him. I'm worried about what's
going to happen to him. Lt. Commander Data: We may never know what
happens. If they succeed, we will not even realize that these events
There are many, many other instances of time travel in TOS and TNG, and moving forward into DS9/VOY/ENT it even becomes a critical aspect of the storyline, so much so that a Temporal Prime Directive is eventually brought into the franchise as a means of addressing the convenience factor. After all, if all you need to do now is configure the warp nacelles to emit chronometric particles in tandem with your warp field, every Starfleet vessel is essentially an interstellar DeLorean, so nothing has to be impactful or meaningful. Data dead during Nemesis? To hell with that; let's just send one Enterprise a week into the past for the next 26 weeks until the Scimitar is assaulted by so many Enterprises that it never even leaves Reman orbit.
Here's the problem, then - introducing JJ Abrams's script into the mix takes the majority of the rules set up to try and address this, and just tosses it in the trash. It not only creates a separate set of rules in an alternate universe which somehow brought Old Spock over from the original universe, but if it is to be accepted as canon actually makes Spock into a lousy scientist and even challenges Data's knowledge of temporal mechanics.
After all, why bother traveling through time for all those whales if it's just going to create a new universe where the only thing that's different is Gillian and two whales aren't in the 80s anymore? Why bother with the Borg time traveling to April 2063; just casually side-step the temporal wake, remain in your own timeline; apparently the Borg just ragequit and signed onto a different server where the difficulty settings aren't quite as high.
Unfortunately, this is one of those things where they cannot both be equally applicable: one method of time travel is a ticket to Retcon Central, and the other kind is spawning multiverses.
Whether or not there is a multiverse in Star Trek is not the problem; there are indeed parallel realities that come up once in a while, most notably TOS's
Mirror, Mirror and Worf's cat-assing around from the TNG episode
Parallels. Time travel isn't really the right key for the lock, though.
Of course the real issue here is that it's just a reflection of the reboot writers' lack of attention to detail regarding their subject material, and it's one of dozens of differences that are tangible (and IMO, problematic at best) in JJ-Trek. If nothing else, the three movies that play with time travel are easily accessible and go a long way towards indicating that they are "setting things right" instead of "copying and pasting in a new universe". So really, if you're going to play it close to the vest, Old Spock goofed things up pretty bad when he Jellyfished back to the future. Maybe in New Trek time travel is a nice and tidy method for sandboxing the reboot, but he left from the Old Universe -- and it's pretty clear that it's tantamount to Control-Z in Old Trek. If a new universe was indeed created, it came at a staggering cost.