I MT almost everything in the game and honestly when ever I see this statement or something along this lines I can't help but slightly agree.
Myself I always take the time to help new players with content and explain each boss before jumping into them. I also take the start of the dungeon a little slower then normal just to see how the healer is and if he can keep up with larger pulls then I amp it up for the group.
Unfortunately I have been in too many low lvl dungeons lvling up alt jobs and seen exactly what you talked about but on the flip side I've also seen it in every other class in the game. The big difference is in the end game content most of it is much harder if your tank isn't doing his job.
Besides my PLD I also play BLM all the time, I love to nuke things lol but that does allow me to see different sides of the spectrum. The lower level dungeons can be very tough if someone isn't geared even slightly for it or if they have no idea what they are doing.
Now I do draw a line, like I said I always try to help players out in my group. I will pull at a good speed for the dps to take things down and for the healer to feel comfortable. I also make it a point to help mitigate as much damage as I can to myself, between Stone skin, CD's and stuns. All while explaining as much as I can to players that need help. If I see a DPS class besides PUG or NIN I havn't lvled them up yet and I see they are struggling with something, maybe dodging or maybe their DPS is wayyyyy to low like 200 points under where it should be then I tend to try and offer advice on skills that can be used and in an order to a degree.
Like I said though I have a line though, if when I'm trying to help I get a veryy rude comeback or a know it all player then I just shut up and don't say anything unless they mess up. Once that happens I kill them with kindness and just point out what they did wrong and how they could fix it useally with a winky face, and I will do that every single time they mess up during the run.
There is no reason to be rude to someone else in game, we all play it to have fun. If someone is being rude just go down to your chat log and turn off party chat. If you can't see it then you wont care honestly and as long as you know your job that's all that matters.
It's all up to you to either feed their fire or you can ignore it, maybe listen to some music and chat with LS or FC friends and ride out the falling apart wave. Most times if one player is being a real pisser to you then they are annoying the other players in the group too, they just aren't letting that one player ruin their bad time. =)
Now to the fun part, if your a dps and the tank is really bad you can do things to just annoy him a little more, say attack the mob in a group he isn't holding aggro on with as many cool downs as you can and then pull the hate and just politly ask the tank to take the mob back. ALWAYS add a smiley face in there lol and or a /p macro with something like Time to power up to over 9000!
If your a healer you can just chose to sparsely heal a toxic player, if he dies and it's the tank i'm sure in the lower end dungeons your dps can MT np almost 90% of the content.
and last but not least if your a tank and one of the dps is being really toxic to the group then just let him rip hate on one monster and watch him drop a little and get nervous. No reason to let him die unless ya want too, I personally like to wait till around the 200 or less hp mark and then throw cover on him. Just to keep them in line and show that particular player that they need a tank. =)