Amusingly (or not), doing pre-made instances with just tanks, just healers, just RDM or SMN is a pretty common silly challenge players do often. Competent players can completely bypass the holy trinity system if they really want to, but these particular jobs have skills that allow for it.
RDM is powerful enough to do stuff like solo all 200 floors of PotD with an expert player behind it.... but it's simultaneously pushed out by meta once you hit farm.
