The problem with the many worlds idea is that there's no free will. We're only seeing the universe that results when certain decisions are made. The person that exists in each universe is the result of a preveious decision. All decisions are equally valid. Each decision creates a new universe.
There is always free will, just an infinite number of realities for every free-willed decision ever made. You are right that all decisions are equally valid though.
I hate to break it to you but there is no free will in any event. At least no-one has ever come up with a rational methodology for it. Well, unless you are a compatiblist, but that's the poor man's free will.


As for whether decisions are valid, that has always been a matter of what criteria we are using. So it seems to me some decisions will still be more valid than others (in terms of their end result), its just that for every "right" decision you make, someone who looks a lot like you will make a "bad" one and visa versa.
But to address the original concern, as I mentioned elsewhere, it does indeed look, from a past draft of the script, that the many worlds business was a late addition (for the obvious reason) and some lines in the script may not have been altered to reflect the change.
Edit:
There has never been anything at stake in a Star Trek movie that I actually cared about.
Now we know why whales are extinct in the 23rd century!
