if you are running a dungeon as a healer or tank, and an item drops that can improve your ability as said role, then of course you can need on it regardless of armor type imo.
I am wearing turquoise dress right now. Bis for all healers in this phase. Including plate wearing Paladins.
You can always need any upgrades no matter the armor. That stat is only important for tanks and in pvp. You should ofc be respectful and not take caster bis and such even if its an upgrade for you unless its your bis also.
That attitude was such in vanilla by the way. And for that I am thankful that classic has changed from. Nowadays people understand what is in a stat and that to perform better one needs also some gear.
In classic you don’t get beneficial stats outside more armor rating from different armor type gear, unlike on retail where you get stat boosts from wearing most suitable armor type for your class. So healer paladins were healing in cloth gear, for instance.
If you are healing instances and healer gear drops, need away.