It's a good point that - regardless of what the rules say, if's largely up to you and the other player how you want to handle it. The nice thing about having a rule is that you can always point to it and enforce it if you feel it's necessary, you can always make the judgment call to cut the other player a break if you think it's warranted. It's just nice having a rule to cover it so that you have a starting point on how it should officially be handled.
Nobody wants to think the worst of their opponents, but there are bound to be situations where it would be tempting for anybody, seeing some of the moves play out on the board, to change their move from what they'd actually meant to set the dial for, seeing that it would give them a much more favorable position. It's just nice to have a rule in place in case of potential abuse of the situation.