Healers may be the most required or useless depending on availability and need for healing. There is also a tendency to forget the supporters and those who generally helped you get where you are, instead thinking you made it on your own merit or that what they did was less important. If you have someone carrying your shit, be grateful for them. Carrying your burdens stops you from getting ruined by a million different minor issues you never encounter with a helper there.
Healers likewise can make people immortal depending on the setting. A good healer in many mmo's will guarantee the lives of everyone in the party even if they are less than they should be. You can generally go in without anything except a healer of some sort. It depends on balance but its really difficult to balance healers in mmo settings which leads to them being either free healing potions or required above all else to do anything. Sometimes all you need are healers because they also damage since all classes must damage.
Anyway the healer being looked down upon is simply a combat strength based value system. If a healer can't fight it doesn't matter how many miracles they can make. They would need to literally resurrect people to have any hope of respect, likely in religious settings. Thats because your measuring them on something they don't specialize in. Fish suck at climbing tree's, therefore they are failures.