I agree with you to a certain extent, and I did watch a video of a run of the dungeon beforehand from a PLD pov to get familiar with it! But on that particular boss, the tank just literally has to stand there and take it. I could see something going on with the crystals that the DPS were handling, but there's no way that I could have given instructions to the new players that were any better than what the healer was giving them! (and was pretty much discarded straight away.)
It's actually why I've been working on levelling up both WHM and SCH/SMN - so that I can learn the actual boss fight mechanics to explain to newer players. On the Diabolos fight - I was aware that our healer was selecting a door, but my instructions were to "tank/spank, and don't worry about anything, I'll take care of it. Oh and stun Nightmare".
I understand the fact about being the team leader, so to speak. But I find it strange that in FFXIV dungeons at least, tanks are the least qualified to guide anyone through anything that's not a "don't stand in aoe, kill boss" fight.