Ok, here's something useful to the discussion then:
I don't want someone I'm sharing with sharing out my entire library to any Tom , Dick or Harry. PERIOD!
But, let's put this on the other foot for a minute... You are sharing your library with me. I have Home Users set up for both of my "Home" areas. Western Washington, and Alaska. Users in Alaska use my media when I'm in Washington and the server is located there. I move back to Alaska, the server moves with me and my Home users in Washington use the same media but now relocated to Alaska...
You share with me, and I share your media with my Home Users. These Home Users access your server when I'm traveling and when they do, they take the full bandwidth or full transcode ability of YOUR server to view your media on their devices. And you would be cool with this? What if they did this when MY server was also set up completely, but I didn't have the show they want to watch? What if their use of YOUR server starts to affect YOUR ability to watch your media remotely, or even locally?
Their hardware would have the limits I placed on them, or the settings I want them to use, but not what you might want them to use. And then you would be calling ME to get me to change MY settings, even though I'm not the one doing the watching....
The bottom line is, this would be, IMO, a BAD thing. Sharing the sharing would need some sort of controls in place to come live as the ability to do so came online, otherwise it could be really abused. (And certainly would be.)
People are already starting to abuse (IMO) Home Users beyond what it was originally designed and intended for. People are asking to extend the limits beyond 15 users so their users "get the same experiences" as the main account does. People are asking for how to get their friend's Xbox set up without needing their own Pass account. They are being advised BY NINJA'S to add these friends to their Home User accounts.
Sharing a shared friend's content would also be abused very quickly, I think. And there would likely be no controls placed on this for a considerable length of time. The tools we have been asking for and desperately NEED haven't shown up yet. I doubt this would get implemented with those tools and controls in place.
I'll leave the discussion with one last comment.... Who are you (or anyone, really) to decide who, how, when or what of MY media gets shared with whom? Isn't that MY job as the Server Administrator?