Depending on what you're doing with it, PLD has a few different subjobs it can use. For example, in Legion /SCH and /DNC are commonly used to help with support, since the Paladin is often on its own to hold the mobs the rest of the group isn't fighting, so it'll need to take care of its own Erases and -na's, not to mention keep up its own healing. /SCH offers more efficiency in MP usage, as well as Erase and -nas, whereas /DNC gets the benefit of uninterruptable Curing Waltzes which can be cycled alongside your Cures to get your HP back up quick, Healing Waltz that can remove most status ailments, and with all the TP you'll be getting from shield blocks (thanks to Shield Mastery), you'll have a decent stream of TP coming back to fuel those Waltzes.
Warrior as a sub is pretty prevalent still for those other situations, for the sake of Provoke and its offensive capabilities. For more solo oriented things and playing around I tend to use /RDM for the Fast Cast and the Refresh, though with Paladin gaining Phalanx natively nowadays, it isn't as potent of a subjob.
There really isn't much use that Ninja has as a sub nowadays, since most mobs that have the capacity to hit you hard enough that you'd want shadows often have AoE attacks to them, rendering the shadows useless. You're also going to be using a shield when it comes to tanking, so the Dual Wield aspect is also useless in that regard.
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