When was all this post-modernist non-sense ever helpful?
I don't know exactly what you're referring to.
I would argue that a belief in the absolute "goodness" of an action can be a very dangerous thing in the wrong hands. It never hurts to try and understand other perspectives, whether you ultimately agree with them or not.
Let me give you a few examples, then:
Science and technology improved the life of billions of humans - including ourselves.
Ethics such as 'slavery is wrong' did the same for millions.
When has post-modernist non-sense:
~'all points of view are valid/justified', ~'scientists are biased/science cannot prove with 100% certainty its conclusions - therefore science is flawed/useless/whatever' + thousands of pages of obtuse text,
ever helped anyone?
BTW, utilitarianism does not claim the absolute 'goodness' of an action. On the contrary, it claims an action may be good or not depending on the situation. And then there's the yin/yang argument (any good deed contains some evil).
As far as I know, some deontologies try to have similar enough views.