What would be the purpose when we can already change jobs if we need to be tank or healer or DPS for whatever reason?

My experience is people ask for these types of changes when they want to be more efficient at soloing content without having any concern about the impact those changes would have on group play. Such changes would lead to homogenization and homogenization leads to people getting bored because there's nothing to switch to when they're in the mood to do something differently.

It also leads to a meta where only one class job gets invited because that job is seen as the best overall at doing it all.