Now personally I think healers should DPS, but if you don't, you do you, it's your choice.
But where did people get the idea that the harder the content, the more you should DPS as a healer, and therefore healer DPS shouldn't be needed in casual content? I see this as a very frequent argument when arguing that healers don't need to DPS in casual content.
It doesn't actually make sense. It's much easier to DPS as a healer in Sastasha than in A12S. In Sastasha you can take a nap while the tank is tanking and by the time you wake up the tank is still at half health. In A12S, you really need to plan your cooldowns and mitigations in a flawless way in order to even have 25% of the DPS's dps, outside of add phase padding.