Composer Intentions?

I have noticed that is plaguing the modern opera scene is the postmodernist approach of "How can you tell what the composer's intention was?" which is the artistic variation of the postmodernist quote “How can you be sure of anything?”  If you want to see an unhealthy helping of it, search "intention" and "composer" at what is unfortunately the most influential site about opera today, Paterre.

Here's a notable quote illustrating of what I am speaking:
"Even we dusty academics stopped talking about 'composers’ intentions' long ago. They’re unknowable in any final sense, the phrase is always introduced as a false appeal to authority, etc. etc."

I think that the composers' intentions concerning operas should be considered in the light of the material available. It is impossible to say that 'intentions are unknowable' without having to necessarily discard *everything* from libretto to score in one fell swoop and just abandon it all. You either have a way to glean the composer's intentions through the material he left behind (some more clear than others) or not, on principle.

If it were not possible, on principle, to glean meaning here then all art would fail to move and communicate, it would be simply noise, color and shapes (or, in other words, post-modern art)- form follows function.