When it comes to time travel stories (in any context, not just Trek), I've found it's best to simply enjoy them (if they're entertaining) and ignore all the implications. Otherwise, it just sucks the enjoyment out of the story.
It used to bother me (in my adolescent days) when time travel was used inconsistently within a particular "universe" (comics, movies, TV series, whatever) and when implications like some mentioned in this thread were ignored. Now, not so much. Probably because, over the years (adolescence was a long time ago for me), I've become accustomed to seeking immediate enjoyment from pop culture (any extra "thinking" it provokes is a bonus, not a requirement) and looking to other things for actual thought provoking discussion or contemplation. YMMV
I hear what your saying. But as college professor, if you had a student writer who insisted on writing about the same subject - over, and over and over - and used the same narratives to solve plot problems in his/her short stories you wouldn't eventually tell that student to get some new source material?
Time travel is even more overused than the Borg as a plot device in all of Trek - and that's saying a lot.