Not all AKEs require contributory behavior to guarantee their security properties, but many do. Your suggestion is pretty close to why; if you're using plain sc

Not all AKEs require contributory behavior to guarantee their security properties, but many do. Your suggestion is pretty close to why; if you're using plain scalarmult as opposed to an AKE with fancier properties (say, 3DH, HMQV...) in the context of e.g. box, it's the sender that picks public keys anyway, so they don't care. In the context of most transport encryption, you presumably do want to prevent those keys from being selected by either party. That means either filtering those keys, or, perhaps better, using a fancier AKE like the 3DH in Signal/Noise or HMQV or something.