Let's explore the following scenario. There is an asymmetrical feature F, with a clear client side and provider side. The client side of the protocol is activated by feature X. The provider side of the protocol is activated by feature Y.
Some nodes want to be a client, but not a provider, and want to connect only to nodes that are providers of this feature. They obviously turn on feature X. Since they are only clients of the feature, they cannot turn on feature Y, so how do they require their peers to support feature Y?