Being nice is about not being pointlessly rude, and about not getting worked up about something you can't control, and not turning an environment ugly, or contributing to one that already is.
You can't control other peoples behaviour, whether they were lazy, or just did things backwards because they didn't know better. You can rant and vent and complain all you want, but it won't make a difference. People will still make the same mistakes in rapid leveling, and most of them won't even see this kind of post.
For primary roles if you followed the crowd and shouts, you might think "this is the way to level up, I will just skip quests until I get to 50" (I have certainly run into people like that. they just did what their friends did). And for secondary roles, a lot of people are convinced that because they found their primary role easy, it would be just as easy to jump into a different role. So they level fast and dive in. Sure most of the time they were wrong but that doesn't mean you should insult them, or yell at them, or give them no leeway at all.
In my own experience usually they will realize very quickly that they are in over their heads, and if they don't, some polite phrasing goes a long way. You suggest for example "you really need to learn <skill>/<skill rotation> before you can run this dungeon. There isn't really enough time to learn as you go, and you don't want to keep wasting time queuing for it. Maybe you should try queing up to <other dungeon> first before you try this one again"
Then you.. Move on. Sure if you don't mind, and they are willing, you can walk them through what you can before you quit but if it isn't worth it because you really can't do anything, just let them know, and leave.
That is all you can do. If they don't listen? Fine. You said your bit and moved on. If they do? You made the game just a little bit better for the next group who runs into that person.
"Don't deserve sympathy" as an answer to "be nice", imo is right up there with saying that they do deserve vitriol. Which is definitely invalid. Nobody deserves to be insulted or ranted at for mistakes. If you can't work with them, tell them why nicely, and leave. Really, that is it.
One of the worst things I have ever come across was queuing in progress into Brayflox, and having the DPS insult a tank so much the tank just quit. Nobody gave advice, nobody tried to train them, nothing. They just heaped constant insults until the person quit. That person never learned from their mistakes, they just learned people in this game are rude and unforgiving, and unwilling to work with new players. Be they new to the game, to the role, or to the dungeon.
So yeah, I still stand by my statement. Be nice, give advice, and move on. It isn't hard, it doesn't hurt you more than what currently happens, and it is much more likely the person improves and does the right thing later. And if they don't, so be it, at the very least you didn't let yourself get worked up about something you can't even control.
Also, there is a great quote for this: "Holding onto anger is like drinking poison and expecting the other person to die."