Think of it this way: your group's end goal is to do as much group DPS as possible, to (in turn) kill the boss before the enrage occurs. You aren't a WHM/AST/SCH when you're a part of a raid group, you're a piece to the puzzle. If you're a WHM/AST you have really great tools to keep your group topped up, so that is your primary job. SCH, meanwhile, has great DPS abilities, shields, and a fairy. Since it can do 2/3 of those things in Cleric and only needs to come out during tank busters/raid damage for the other, a SCH's primary job is to contribute to raid DPS.
Look at tanks, for example - you have two tanks in your raid, yet how often is the OT in tank stance? Not very often, if at all. That's because his job, until he needs to tank, is to contribute to damage. You essentially have "main healers" and "off healers" in the same regard.
All that said, if the off healer is in a situation where they need to heal and they instead choose to DPS, then they aren't performing their role. "Off healer" doesn't mean you're just a DPS, you're a DPS with healing abilities that need to be utilized. So if this is what you're experiencing, I understand where you're coming from, though in that case it sounds like you're healing with a bad "off healer".
