I have 10+ users who stream off my server, each person have their own plex account, I do not share my main plex account.
This is pretty much how I have it setup:
Friends and Family have access to Movies and TVShows.
I have access to my 4K movies and BluRay Rips (HQ).

I did in the past have folders for G-PG13 and a separate folder for R rated movies but now I put everything in one folder.
The simplicity of the app and the authentication is fine by me, I enjoy it. I chose which accounts that are tied to my server and which folders they can see. Having to put in a username/password when I pull up plex is annoying and my wife would hate it. I much rather control what they see server side (plex.tv--Servers--Edit Sharing)
My wife has her own Xbox One profile (hey they are free and apps are no longer behind a pay wall!). Her plex profile/account is tied to her folders.
I have my own Xbox One profile, I can see any folder.
The Xbox One profiles can require passwords to sign in, just like a computer account (they are tied to your Hotmail/Windows Live, whatever). They also support 2 factor authentication.
As far as having guests over and hiding stuff from them...then thats a different matter...
This isn't just about the people who live with you. It's about all those you come into contact with.
For example. if my parents come over and I have to switch to a different xbox account to load different Plex movies.
They are going to wonder why I have to do this in the first place. Seriously, it would be odd for them.
I love my parents to death. But they are very curious people. They would most likely wonder.
Why am I switching accounts when they are over watching movies. I know, because I could respond.
Well, mom and dad. I just don't want you to see that certain content I have.
You know, it could be pics of me and my friends going out drinking.
My parents wouldn't care to see or know stuff like that.
That's why I would want that to be private and no questions asked.
I don't want to be questioned of why I am switching accounts when someone is over.
It looks like you are trying to hide something.
As I said, you guys might disagree. I am cool with that. I really am.
But it should not be hard to have 3 authentication options that can make us all happy.
1. No security - So that "annoying" login prompt you were talking about.
In this option, it would not be there.
2. Xbox One Account Security - This will tie your xbox one account to plex.
3. Local Security - If I turn on this feature. Security will be independent from xbox one accounts.
I can create my own usernames and passwords. Then assign groups of folders that belong to these accounts.
I would like to see this done for the Ipad to. Right now, anyone who opens my ipad plex app can see all of my content.
Seriously, all of it. The local security option would fix this issue.
I think something like this would cater to various people's needs.
It would be a big win for everyone.