Please correct me if I am wrong but I thought this was for sharing your phone with someone else or sharing your phone between two or more Google accounts...
I think Facebook Messenger, Line, and the like will still have access to all permissions even if you switch to a different user and install the apps there...
That being said, guest mode is really nice on my nexus 5 so my curious friends on iPhone can log in to their google account on my phone as a guest and test drive android.
I think the idea is, you create a dedicated "sandbox" account, install apps in it that you don't trust that want access to calendar, contacts, text messages, etc., and then don't put any real data of those kinds in the account. So, they still have permission to see those things, but they don't see anything when they look.
Note, I have not looked deeply, so maybe it doesn't work like I said. I would not expect multitasking to be very seamless with this method. Also, I know there are some permissions that have "cross-user" abilities, so maybe there is still a way to accidentally allow an app to access your real data.
I think Facebook Messenger, Line, and the like will still have access to all permissions even if you switch to a different user and install the apps there...
That being said, guest mode is really nice on my nexus 5 so my curious friends on iPhone can log in to their google account on my phone as a guest and test drive android.