It hurts to say as a white mage, but Grimmy is right. Now I actually do remove status effects and such if I'm in a ninja-tank party, but it is certainly less engaging than if I was in a paladin-led party. I get bored. So, I drink a Diet Coke or something while I wait for the next pull to haste the ninja and such. In a high level party with a ninja tanking, I would suggest the following for a
Sploitz Party
Ninja/Warrior: Main tank. In addition to bringing elemental enfeeblement to the battleground to exploit weaknesses, the ninja participates in skillchains with katana.
Emergency - Mijin gakure. Massive damage at no xp loss so the ninja can eat a Raise 1 from red mage or summoner (or just use a scroll, gorget, or hairpin).
Dragoon/Warrior: Support DD. The dragoon will provide (even pre-update) signifigant damage with polearm attacks and weapon skills. The warrior aspect will act as a first 'voke for the Dark Knight's SATA on the Ninja as well as have the wyvern attack the mob's elemental weakness (enhanced by the ninjutsu).
Emergency - Right now the wyvern is it, but after this next update a dragoon will be imbued with strength and special powerful jump abilities.
Dark Knight/Thief: Support DD. More massive melee damage not to mention SATA with a great sword or scythe. In addition, the mob can be crippled by absorbption spells. Treasure Hunter doesn't hurt either.
Emergency - Blood Weapon is like a self-Benediction, especially if used in conjunction with Soul Eater or Last Resort which will also cause the mob more damage.
Red Mage/Dark Knight: Battle mage. With a dagger or sword, this red mage is right up in combat adding more to skillchains and melee damage (made more effective with "En-" spells). Red mages also do the other duties the job requires.
Emergency - Chainspell to nuke, which works even better if Convert is available.
Black Mage/White Mage: Raw elemental magic for magic bursts as well as elemental enfeeblement spells (to further cripple the mob) such as Rasp. White mage sub gives some healing and status effect removal support. Let's not forget Sleep, Warp 2, Escape, and such.
Emergency - Mana Font. Unlimited power. A deluge of nukes on an already enfeebled monster.
Summoner/White Mage: This is pure elemental melee (with some magic too). Summoners also have unique party buffs and monster enfeebs. As the tank is a ninja, there isn't much healing needed and the red mage can always cast high-lev prot/shell as needed.
Emergency - Astral Flow. I don't need to say anything more.
See why it is difficult for me to bring this up? This party would work so well and all without a white mage. I suppose if you wanted to tweak it a little bit you can have a BLM/RDM for fast cast. This may even make another interesting "battle mage" because the black mage can use Phalanx and Stoneskin with its dagger (or even scythe!) skills. The red mage could also sub black for a bit more mp or the ability in an emergency to chainspell other spells, but at its cost to be effective melee.