I had fun with this the other day.
Someone in my LS I joined - with whom I have never played - was asking if it would be good to get his Relic +1 or go for some more armor instead. Found out he was (a) newer to MMOs in general, (b) never played a tank before (it was a PLD he was asking about) and (c) was having issues being booted from dungeons "as a bad tank".
I asked him about aggro issues, and he said that was his main situation. Saw his gear and he was basically an i80 PLD. So, instead of asking him what he did in dungeons, I changed to WHM, partied with him, cued up, and went into WP.
We paired with two i85+ DPS, melee and BLM. I told them right away that this was purely a training run, may take the whole time, and if they aren't willing to wait for it all, they should leave. Both responded they actually had no problem with that and stuck with it.
In the beginning, he could only hold 1 of the first two TBs without me or a DPS pulling one away. So, a little direction, and that wasn't an issue with those 2 mobs.
Well, about 45 minutes later, NO ONE could pull aggro from the PLD even with 7 mobs on him at once. In fact, that was the last training example: "Ok you two DPSes, do whatever you can to pull aggro of any mob off of him. Let's see if he can do it." And me included in that with overhealing and DPSing.
I'm not trying to put myself in the limelight; its a point of some of these so called "bad players" are given some personal direction that doesn't take a lot of time and effort if done right, and as a result, find out that they can actually do their role quite easily. OF COURSE they have to be willing to learn, and also don't treat them like they're ass-hats. If the last two points are in, you may find yourself with another great co-player when you want to team up.
Maybe 45 minutes is an extreme case. Maybe it is too much to ask of people in general. But the way I see it, I took someone who actually started hating tanking in general, felt like he was letting a LOT of people down, and turned him inside out to the point where he's rocking it. Besides, we need more effective tanks who like that role. Now we have at least one more!
TL;DR: Someone as a tank had pretty much given up tanking. 45 minutes later, going through WP, he was turned into a beast. If you train them, they will be awesome. Try it once. You may find it fun.