An actual unchanging truth is extremely beneficial for planning. But if you're planning with open eyes, you know that what you actually have are probabilities based on what you know when you make the plan, which I believe to be GP's point.

Anyway, there's no other definition of "truth" that isn't a mockery of the word as used. In practice you work with the closest you can get at the moment, but you don't redefine your terms down to match.