The healing of Marrowgar is intense because the fight of two things.
-Misconception of the "Saber Lash Cleave" in conjunction with the coldflames. If your tanks have full threat, then you should be able to move into range without fear of being cleaved. But don't get me wrong, that lash hurts plenty and constant heals are needed for it. Ideally if you're waiting 'til 50% to cast, it's probably too late for both of them because each lash takes 30% and within the time a Light pops, your warrior tank will probably have cease to live by then (specially true if you take into account "Beacon Heal Lag").
-Whirlwind should be handled by the raid healers if at all possible. Your big heals are also the most draining. If you find yourself dipping, of course prioritise yourself but your main priority is the tanks before anyone else. If you feel like you can't assist with small heals while you main task fails, stick with the main.
It's basically a heal and tank check. One to see if the tanks can handle the damage. The other to see how much pressure you can deal after being a "reactive" healer for so long (Remember, avoidance dulled many healers perceptions on when to heal now or what to heal).
Healing is the source of life. Without it, you're left with death.