Because if they die it resets? Healing them is part of the mechanic. (And mushroom always heals for a few million, mine heals for 1.8 mil a pop)
erm... I'm not sure the mechanic is healing the NPC's so much as it is preventing them from dying. They have rather too much health to worry about healing them outside of incidental AoE's. Especially when you take into consideration that the attack you have you actually worry about is a percentage based attack.
Fun thing on Galakras, keep the npcs healed up with a rejuv or something and watch your healing go up. They're always injured so it's pretty much 0% overhealing, and you can do it smoothly with mouse over rejuvenations. Helps put the disc priests in their place as well.
So, you're saying Disc should feel bad because you're padding meters? Good to know =)
To the original question, if you can hit the break point without completely gutting your regen and ALL of your mastery, yes... it is absolutely worth it. My cohealer is a resto druid. He just recently experimented with the 13k HBP and SotF. Now every SotF buffed WG is almost a mini-tranq. The amount of healing he can put out with that is amazing.
I know most resto druids are able to hit the 13k around 545-550ilvls so you are right in a place to start looking at that. If you have a hunter, they can bring a spell haste pet too, which may open up your options a bit. (I want to say the spell haste pet is a spore bat, but not sure)