Sync file ownership most needed not for end users, but for system administrators in corporate area. For example, I need 2-side sync /home folder in about 20 computers, with about 50 users. All computers have same user/group list (using shared ldap user list), so all that I want is to keep uid/gid of files when I do sync.
Running about 50 clients from each user will eat all memory and cpu, this is bad solution.
So the good solution is run syncthing as root with sync uid/gid feature.
I understand that this will be some potential security breach with this mode, but if some hole will be found, it will work for single user instance too, so it must be fixed for all users, not only for root mode only.