"Harmful", yes :P Like, I'd be like you, I wouldn't care... IF it didn't mean that I had to work harder as a healer\tank to keep up with the pull. If other people aren't doing that effort, why should I? It's not like I'm asking you to exert yourself because I'M exerting myself. I'm saying that these things are baseline, and that not doing them IS rude. Because the only reason I'm exerting myself is to make up for the fact that you are not. That for me is the harm.
It's more than just "oh lol, wacky DPS went ahead and pulled". The tank can grab aggro with one AoE, literally who cares. But dragging the dungeon down because you're focusing on one guy all the time is just going to make the pull last longer. And if it's in a dungeon that hurts a lot, then... yeah, no, you're not just being rude, you are being harmful.
idk if people remember Bardam's Mettle back when it was relevant, but remember that pull before the 2nd boss?

Imagine a new healer having to deal with all that but their DPS decided to only burst down one mob at a time.