I install everything as root, unless it is software that I am not sure of (i.e. alpha or beta software, especially those that make network connections). If you install as a user, then other users cannot use whatever you are installing unless you change the permissions. I prefer to install as root, so all users can access the programs. If I only want certain users to be able to access a certain program, I just make a group for those users and set the permissions for the program accordingly.
I am not sure if installing as root is the best procedure, but it is almost necessary for RPM install and compiling/installing from sources.
Must the installer be exclusively root only for the "all users" availability?
No. You can set the permissions of the files/directories to allow access to whoever you wish.