It seems to me that you think (the one who started the thread), as it is so frequent to find Duty Roulette overgeared players everywhere, the normal situation is being overgeared and marking is not that important. In a normal situation where the group is not overgeared, the sooner the first mob dies (and second, etc.), the more probabilities of things going well, less damage received, and probably things go quicker (the less mobs there remain alive, the quicker they die, less damage received, the more a healer can DPS, and healers DPS is strong if they have the possibility to use it.) I think marking 1 might be enough so that the effort concentrates in that one, that might be important (the right Tonberry, the healer mob, etc.) or just convenient for quick deaths. Once that 1 is dead you can mark 1 the next to die on the list. If you manage to mark quickly, mark more numbers. Sometimes it is not necessary, in places where it is evident all know well the place and wiping with trash mobs is very unlikely, and the tank will have no problems to keep enmity.