Playing a healer and a DPS class is almost imperative to tank properly, and viceversa. You need to understand how healers cooldowns and abilities work to take the most benefit from them, like waiting for Protect, SS II, change to Defiance for an adlo in between phases in a boss fight, or position accordingly to receive a Medica in time. And also to know when the healer needs you to pop a cooldown and when it's not necessary.
And also don't move the enemies at all unless necessary, to not fuck up MNK's positionals or NIN's trick attacks.

Everyone think they can teach new players, but only those with that much experience actually should.