I don't know about the post-modernism thingy.

To the second part: To one practical is to the other unpractical. To one just is to another unjust.

To push certain categories over others just says to me you want them to be in your frame(practical, useful for you I guess?).

And to not give an argument on the usefulness is to hide intention. But I'm not too firm on that yet.

Some categories are just completely impractical, for example the category of "all objects except for my dog and its kibble, and excluding those that came in to existence on an even-numbered Gregorian year." Language is an application of signaling theory and there are lots of statements we can make about it. (For example it would be a bad idea to re-define the word "good" to mean that one above.)

It's post-modernism only if you assume that human beings have no set, general psychological characteristics and are entirely the product of socialization. Otherwise it is pragmatism.

Huh?

It's most assuredly post-modernist because it implicitly assumes that all categories are equal in value (i.e.: that no category is better than another), and that the decision is at it's core a power-struggle (i.e.: oppressive to the loser).

This is, again, a tired and fruitless approach.