What happens when the rocket blows up?
I completely agree with the concept of documenting your opinion in such a case. And I believe that the PM may very well know something that you don't. BUT, I of the opnion this type of political flim-flam is what really can inflate the cost of real quality work up.
The old factor of "The customer is always right" comes to mind. But, at the same time I see little Dilbert cartoons acting out the situtation.
Its a balance folks. You take the poor decisions with the good ones.