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it sounds like it would do far more harm than good to use for light-armored roguish characters, since a disproportionate amount of your AC is going to be lost by going flat-footed (in close melee range no less). 

Yes, that is indeed the case. In fact, you don't just need to be in close range, you need to be in actual combat. So your target is already taking swings at you.
Taunt can pair well with the Expertise feat, but that does make the process even more elaborate - you need to drop expertise to start doing some actual damage.

If a warlock was to equip a shield or prohibited armour, would their invocations just cease to function completely, or would they simply be subject to Arcane Spell Failure chances?