Friends+ instances are about letting all friends of friends to join there, most popular instance type for people just hanging out around and visiting worlds, yet now everyone must remember to always unlock the portal before dropping because otherwsie not everyone can follow you.
If its friends+ instance, I want friends of my friends to be able to join, current default behaviour is super unintuitive, many people didn't even know that their portal is not visible to some people, as everyone expect that a portal to friends+ instance can be used by friends of their friends or just everyone around so they can follow along.
If I would want only my friends to follow me I would drop a portal to friends only instance.