I do mainly run dungeon as whm and having around 14k MP is a lot to DPS with.
I choose to DPS most of the time because "there is no need for the team be 100% HP 100% of the time" i don't care if the tank, DPS or me are hurt... As long as it doesn't reach 0 why would i burn MP to heal ?
At 60 you only need 3k mana in order to keep the tank alive without problem (not counting bene and tetra which are awesome skill)
I always stay around 5k mana dpsing and healing enough, and never had a struggle.
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Oh and of course this is not a way to run 8man content where you have to manage your mana depending of your co-heal
But i do follow the fact that a healer dpsing is not mandatory
Just frustrating to see them standing near you acting as heal bot XD
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Good advice : a DPS who go crazy eat aoe/cleave or stay far away from you, must die as painfully as possible. I have enough mana to heal the tank and unavoidable aoe on the group, not one idiot player playing around xP