@Positronic-Rift Healers basically raise the (TTK) time it takes to kill the player they are healing. Even a player being able to take a little more damage can be a huge factor in the outcome of a matchup.
I think it's good to compare the TTK of killing someone without Healing to the TTK of killing someone with Healing. I haven't tested Sunflower healing too much other than my observations playing as and against Sunflower in PVP, but I believe in Sunflower raises that TTK too high. It takes too many shots and really too much of a commitment to kill the plant she is healing. And if you take off the pressure to reload or take cover while the Plant is still being Healed, the plant has already gained too much health back to make the previous damage worth it, plus the huge constant Healing output that makes killing that Plant take so long.
I think the player base has slowly caught on to that, which is why Healers get targeted so much.
I'm not sure if Scientist needs a Healing Nerf. In my experience his healing definitely can't keep someone alive as well as Heal Beam, even with upgrades like CPR and Power Wash. Out of combat Healing Hose still heals at a fast rate, which is good because an effective Healer should get their teammates back in the fight as quickly as possible.
I don't think Healing needs to be nerfed to the previous Sunflower and Scientist healing (remember how slow Healing Hose was? And I also would argue that the Sunflower's previous Heal Beam had a good output of Healing, at the time it was way too good because of the game's overall super high TTK and it was still better than Heal Beam in the GW games. ).
It is HPS vs. DPS, but I think the clearer question is "by how much should Healing increase the TTK?"
Popcap needs to find the right Healing-Per-Second that doesn't raise the TTK too high to where its too much of a commitment to kill the person being Healed, but just enough that a player can take plenty of additional damage to still make a difference in a fight.
Popcap should definitely look at how good Healing is at saving someone at low Health because this can be a good example of how ridiculously strong Healing can be. Sunflower's Heal Beam makes it difficult to finsih off a low HP Plant, especially with certain upgrades.