Hi,

Repose is useful but you have to agree with your team 1st.

When tanking you have to switch aggro. Healer is getting aggro on all mobs, and you can have AoE DPS going around too, or people landing some dots.

Only way for a tank to build aggro on slept mob without breaking sleep is to use flash.
Best used when you are with a GLA/PLD, as a MRD/WAR might want to use overpower breaking the sleep.

About your MP management, if the tank is geared for the content maybe you have change how you heal? I have both heals and main SCH, they work differently, except the cure 1/physick spam