I'm presently leveling a ROG/NIN and I'm having a blast with it. I've put vitality, strength and determination materia on all my gear since about level 25 and I've been yanking aggro from tanks without meaning to. I honestly didn't pay attention to whether or not the tanks were forgetting Shield Oath (I know that happened once, cause the tank mentioned it) or what, but I had this happen in all but maybe one dungeon/hest that I've run. At first, I thought it was because I was attacking a target not the tanks. No, I was on the tank's target. In instances where the tank marked targets, I stayed on that target in numerical sequence. Still pulled aggro. I even pulled aggro when there was only one target in the room.

You might ask why I put this materia on my stuff. Well, I give the gear what it will take. I tried to meld Dexterity, but it gives zero stat boost. I made sure that I put as much vitality as I could meld so that I wasn't so "squishy". I have received several commendations as a Ninja. I have yet to have a tank say, "Hey, stop stealing my aggro!" I get "Nice job". I'm not trying to tank, I don't pull mobs by running ahead, I drop DOTs on multiple targets, I hit what the tank says to and I maximize damage in my rotations. As was said, what's the problem if I beat down trash and hold the aggro. It's not grabbing the healer and I'm not dying.

Of course, I see a lot of ninjas, and I do see all the things people complain about, especially those who don't get out of the way of an AOE then get dead. I see them cast an AOE mudra when there's only one enemy or worse, when they don't want to attract the surrounding mobs. I've seen other folks raising ninjas and done the same, but I don't think it's any larger a percentage of other melee DPS. I also see BLM's and BRD's stripping aggro from the tank just about as quickly.