Without being too hard nosed about it, if you consider that you are not capable of executing your mission safely due to fatigue, then it is your duty to report it to the a/c commander/authoriser/Squadron hierarchy, not whinge on an anonymous forum about it. If the rules designed to protect you and your crew cover this (which i'll bet they do), then use them, or are you afraid of telling your superiors that they have made an error in judgement? Who knows, maybe they really believe everything is OK at the sharp end. They aren't going to do anything about a pprune thread, but they will surely sit up and take notice if you actually play the Flight safety card.
If the operational tempo is altered because missions are lost due to crew fatigue, then perhaps those that can, will do something about it.
If you are operating within the rules, and still feel this way, either they (the rules) are wrong, or you are!