They have social and visual value, but not gameplay value.

That's my point. Saying they're not affecting gameplay doesn't mean much when they have other value to players.

Doesn't mean much other than the fact that Blizzard implemented a feature that doesn't ruin the integrity of the game.

Blitmap seems to think it means something. Again "integrity" or "mechanics" of gameplay aren't the only value here.

they have perceived value despite their non-existent gameplay value. Oh, and the 100% margins.

You had to pay an artist to make those hats ;)