They have to start somewhere. First get it to work properly for individual accounts and then add features for sharing a photo library (not that wouldn't exist already, just not as automated as you would like it to be and with a image size reduction).
What you essentially want is to use different AppleIDs for different services (ie, you and your wife use one ID for the iCloud Photo Library but for things like contacts you'd want to use separate IDs). This is currently only possible for the iTunes and the App Store which can use a different AppleID than the rest of the phone. But I doubt that will ever be rolled out for all different services, it just creates too much confusion even if for some people it would allow them to tailor exactly how their phone behaves. If you use different providers for different services, then this obviously works but it is not a given provider allows something like this for its services (eg, use ID 1 for folder A in Dropbox and ID 2 for folder B).
No, the only solution that will come will be one using explicit sharing features (as they already exist with Shared Photo Streams). Let's imagine a something under the Family sharing umbrella. Their could be a master ID into which everybody can post photos, the question is whether all people in a family will be happy with sharing all their photos. If not, should people something like different tabs between ones 'own' photos and the shared one (as the Shared tab that already exists). You'd then need a preference setting for whether to share all photos with the shared pool or only selected ones manually. And should everybody be able to delete any photo from the shared pool? Or is that yet another setting?
My point is that simplicity and behaviour tailed to different people's needs is hard to achieve. Not that Apple should strive to do so, but there will always some tension between the two.
