Still confused. I'm usually around 450 DPS for the first add phase and I have at least 2k MP if not more by the time they're dead. Only way I could see burning that much MP is if you are on bait duty for the tail swipes for the other tank (Ruin II spam), but that should be a bard and doesn't represent SMN MP conservation.
In between adds you can usually sustain your MP by not using Miasma II except if RS is up, and not using Ruin II. You can always make up time at the end of the fight by burning more MP on AA/Ruin II spam, M2+Contagion outside of RS.
If SMN could replenish more MP I'd think it's single target DPS would get out of balance with other classes.