I agree that people have different priorities for functionality, but good notifications are pretty much key to staying on top of things and getting work done, in almost all problem domains.
I guarantee you that no one using Fibery where entities need to be observed by someone other than the actual assignee thinks that notifications are “good enough”.
Watchers would be a huge improvement for hopefully very little work. You’d clone/refactor the assignees functionality, remove a couple of bits from it (it won’t be used for avatars badges on summary links, etc.), call it watchers, and be done.
Not only would it massively improve the user experience, but it seems like a relatively easy task that is unlikely to result in compromises or knock-on difficulties elsewhere in the system. (Of course I may be wrong here - I am not a Fibery dev, just an experienced external dev looking in.)