I'm not sure you understand what we're asking for with regards to shared notebooks. First of all, 'user protections' are irrelevant here - the notebook has already been shared with the collaborator. Key word here is 'collaborator'. Granular permissions with regards to what collaborators can or can not do would be a welcome feature but at the very heart of what we're talking about is the fact that collaborators can, in only some circumstances, add tags to a shared notebook. A collaborator can add a tag to a notebook if that tag already exists in the owners collection of tags but a collaborator can not add a new tag that does not exists in the owners collection of tags. If a collaborator can easily delete the entire contents of a note that's been shared with them (what 'user protections'?), it seems more than reasonable that they should be able to add an additional tag/classification to said note.
In addition to this shortcoming, shared notebooks exhibit disparate functionality across Evernote's products. The desktop application allows you to search for a note that's been shared to you based on a tag. The web portal and iOS applications do not support this - currently, as a collaborator, if you search for a tag that's been applied to a note that's been shared with you, no results are found in the web portal.
Evernote is a productivity tool that's frequently used by people to collaborate. If Evernote is going to make the push for it's users to consolidate the number of devices they use or pay up for a premium subscription under the guise of improved functionality and additional investment in feature sets (go figure - moleskin notebooks and attache cases diverted focus from the main product) then I think that it's perfectly reasonable to expect a consistent experience across each Evernote product as well as a robust feature set that allows people to productively collaborate.
Evernote's lack of willingness to address this bug - which has been brought up in multiple forum threads and support tickets - is a market opportunity for another player in this space. Currently, like many other people in this thread, I've invested a lot of time and effort curating notes and notebooks in Evernote. Unfortunately, there has yet to be a super attractive competitor to come along and motivate me to switch but that's changing. Notebook, Betternotes and Paper (in addition to many others) are all making a play in this space and it's only a matter of time until one of those guys has a mature enough product to warrant the switch.