It's a cultural thing I suppose.
To me, using something like Duty Finder is entering a sort of social contract in which we attempt to achieve a shared goal (the dungeon/instance). Because we're operating as a group, it falls on each player to, at the very least, put forth a basic understanding of how their role/class operates and carry their share of the "weight" in the party. It's simply the responsible thing to do. This means things like combos/rotations, CD cycling, DPSing when no healing required, etc.
Anything less than this basic game play shows that you have entered the group with the intent of completing the goal without carrying your share of the effort, and therefore you want to leech off the efforts of others. It shows a severe lack of respect both to the other players and their time.
Sometimes there are players who just don't understand something, and that's fine. If they're willing to learn you can instruct them on the issue(s). But often times they're not and will either ignore any advice or actively deride you for giving it. In this case, the mask comes off and they've shown they don't care about you or your time, you're just a vehicle for them to leech off of till the dungeon is over. And we should not be promoting that kind of behavior in the game. It's no less toxic than the toxicity of elitists, but still there are people that will give it a pass for some reason.