The problem Ninjas using Goad on tanks in long fights is that the other melee will need TP as well. If a monk or dragoon doesn't have TP, they can't do their job. If a tank doesn't have TP, he has done his job well enough to not have to worry about losing hate and can do other things while he waits a few ticks for TP. You have that full MP pool, might as well use it if you don't have TP.