In that video its actually your whms fault that the drg died. Nobody is stoneskined for nerve cloud. When the party is killing the egg he has enough time to stoneskin everyone because very little damage is going out on the party at that point. You want a change when the fight isn't being properly executed.

The whole mdef thing is just a flavor type thing most likly they can off tank adds better then say a mage. Some people use a drg to tank the garuda ex add because they have higher physical def so they only need 1 tank.