[...]it seems that flexibility in meaning is useful for communication. I don't have to hunt for the exact right word, many approximate words will do, and I can describe new concepts by stretching an old one.

Yes, strongly agreed. This idea makes me want to think of adjectives as tugging a concept-defining region of thing-space in a new direction.

That works especially well in languages like Lojban, where adjectives and nouns are not distinguished from each other. For example there is "blanu" which means "x is blue" and there is "zdani' which means "x is a house", and you can say "blanu zdani" (blueish-house) just as easily as "zdani blanu" (houseish-blue).

[...]saying that a bird must have feathers (a necessary condition)

That actually is a good example of a brittle requirement, in ways that are even more directly problematic than shuttlecocks and bird-shaped staplers. What about a plucked chicken? What about a duck that, due to a genetic disease, never had feathers? What about (NOTE: This example isn't valid re: real evolutionary history) a member of an intermediary species between pterodactyl and modern birds?