The problem with RDM/NIN tanking is that:
a. it is incredibly high maintenance to hold hate - you need to be performing a whole lot of actions on the mob, meaning the other stuff you do as a rdm will likely take a back seat.
b. almost every other tanking combo holds better hate with less effort.
While I think RDM/NIN tanking looks like a whole lot of fun, I dont think it is very ideal - I cant think of a single fight where I would prefer a rdm/nin tank to something else.
XI Wiki

