The important thing for me has always been the writers intent. In "First Contact" we are supose to asume the timeline was fixed and the Enterprise crew went back to to trekverse they have always been in. I don't think this how time travel works in reality,asuming we ever figure out how to do it, but I don't have problem with going with this idea,simply because people want to feel like they have been watching the same characters from beginning to end.
The writers need time travel to work differently for the movie,so they could create a new trekverse without erasing the orginal one so they went with the the theory that a divergent timeline would be created. This is proably how time travel would work in reality but realism wasn't the intent. The intent was the achieve the above mentioned goal. I don't have a problem with that.