It's not that you have to but you aren't putting the effort in to increase the effeciency of the group. It's like being annoyed when tanks or dps don't use cool downs... It's available to them but not required to beat most content. I mean I don't have to use berserk when I'm a warrior but seriously why not?

Also I don't think anyone is arguing that a SCH or WHM should dps at the exclusion of required healing but so much of the content in this game is so light on healing that it feels like it was almost designed to give healers the room to dps in inconsequential content like over geared 4 mans. Not like I'd kick a healer for standing around when there's no healing to be done, but I'll view them as not contributing as much to the group as the rest of the team. But that's not really much of an issue since I'm usually the one healing and making sure there are SCH dots all over the place.