Many boss fights in 11 used to have players switching in and out during battle, black mage kiters running out of MP and being replaced by someone else, someone dying and being replaced by someone else, I guess this is why Tanaka originally put this in so players couldn't abuse HNM fights.

There are better ways of doing this like only allowing a maximum number of people into an instance to fight a monster, obviously this has its draw backs aswell in that all boss encounters would have to be instanced.