Time travel in films never makes sense. The very idea of being able to send someone back to change the events which give you cause to send someone back is just completely ludicrous. Even if you could send someone back, failure is inevitable, because past events are past events- you know you won't visit yourself as a child with proof of time travel, because you would remember that.
The "alternate realities" thing is of no use if you can only be conscious of one, as we are, but in the case of Terminator, how can there be a universe where the clueless Sarah Connor is killed by something that was never sent back in time? But more importantly, what use is that to the machines that send the terminator back? None.
In fact, the most disturbing part about time travel is the way that things become so inevitable. Events in the past cannot be changed, and unless we have some weird egotistical belief that we are at the "edge" of time and the universe is being played whilst god holds the record button, then this time is, from any future perspective, the past too. So the coming 10 minutes are as unalterable as the previous 10, and everything we ever do has already been done and is perpetually being done, the universe is simultaneously at every single moment past and present, and nothing can be changed. Which I find depressing.
/xmas random rant