I suppose. But what are the chances that sisters will ever get, I don't know, an order specific special character? Next to none, I'd say. Same for model that says "All tyranids with the [Hive fleet Jormungander] keyword reroll 1s to hit or "All models with the "Craftworld Iyanden" keyword 6" around this model gain +2 movement". It makes sense for marines and basically no one else, as it currently stands.