It's been said in different ways, but the underlying factor here is knowledge -- a recurrent trope in stories that deal with Time-travel is that things must occur as the prophecy / foretelling / whatever says.. But there is wiggle room, in what is NOT said, or what is NOT know.
In this case (avoiding spoilers here), the doctor had to be there.. and people saw what happened.. But the doctor took advantage of the fact that what they thought they saw was not necessarily what they actually saw; it may be a fixed point in time, but if people know it simply by 'viewing' it somehow, then it leaves what is actually happening open to interpretation.
So why River? Because a fixed point in time is like a black hole for probability; it's inescapable.. well, unless you want dire consequences for the universe as we see. Once it was 'seen' (historically, by prophecy, whatever) it was set in stone.. And since River was seen as the agent, she had to be there.. History is inaccurate, in that she had no intent to take the action she did, but what history recorded is what was seen. Could someone else (say a Clone of River) be used? Sure, but the Silence was behind this, and they WANTED what had been seen to happen to occur, so they took every step to make it a (I don't believe I'm about to say this...) retroactive self-fulfilling prophecy. They wanted it to happen, so they took every step to make it happen as it was 'supposed' to.
Again, we are still dealing with an ontological paradox.. but that's nothing new in the Whoniverse.
Synopsis: River was necessary, because in the fixed point that was seen / recorded / whatever, she was there. She could no more be removed from the situation than the Doctor could (humor intended.) The fact that she was not in charge of what was happening was irrelevant, she was there to fulfill the requirements of time/history/destiny/etc.