People can't agree on this topic because whenever it's discussed, each person posting changes the scenario to whichever supports the position they want. Is it reasonable for other party members to ask the healer to do more DPS? It depends on the situation.

a) Healer throwing out an occasional heal, and otherwise standing around doing nothing most of the time. Another party member asks them to DPS. Is that a reasonable request? Yes.

b) Healer is swamped with trying to keep everyone alive due to a squishy tank, poor threat management party wide, people standing in avoidable AoE, or whatever, so there's no chance for the healer to DPS. Another party member asks them to DPS. Is that a reasonable request? No.

There is no right answer to whether it's justifiable to ask a healer to DPS, because both answers have times when they're correct.