Every time I see one of these threads I'm always bent on the issue. Although I haven't played FFXIV that long (I played a lot of 1.0 but there wasn't really anything to do), I've played FFXI for years.
When ever I wanted to do a shout type party, it was expected you'd see sub par performances from any job. The results were always mixed and was to be expected. If you wanted to get things done more efficiently, you'd want to be in an active linkshell. In relation to DF, you're essentially getting the same mix of players. In my experience with partying, the casual gamer usually doesn't go above and beyond often. It's difficult for me to get upset with a player who wants to experience more difficult content but isn't a great player or doesn't really know how to get better. Hearing the argument that there are plenty of guides and videos doesn't really convince me. It takes a good amount of time to do research and to keep up with changes in game. Sure I look up shit, but that's because I want to perform the best I possibly can. The thing with duty finder, is you're letting a function put together the party for you and thus, saving you time with the party set up. And seeing how much it upsets people on these forums about bad players, I can assume you know what you sacrifice with the auto party function.
I've only ran into one party where someone really went above and beyond to explain what a player was doing wrong (Had a pissed off tone though lol). I felt bad for the whm cause I could tell she was already trying her best. Would I take her to a raid? No, but if I really wanted to get something done in any other lower tier dungeon, sure why not.
On the flip side, being grumpy and tired, I was very forward with what the DD weren't doing right. As being the healer, it's hard not to get frustrated when you have 6 mobs and the DDs run in circles rather than bring them to the tank.
I really do want to agree with OP (and others alike), but I can't quite do it. If I really want to complete something, I'll ask my FC and do my research. But if I'm relying on the DU, I'm def not going to set my expectations very high.
If you're doing higher tier content, of course it's not taboo to let the DD knows how they're performing. When it comes to other stuff, I still don't think it's taboo but I'm also not going to let it ruin my day. Cause coming here to the forums and complaining here is doing what?
Now poke the holes in my argument XD