1) Making a tank just because you got a suit of banded is a terrible idea. You can level up whatever character you're currently using a few levels and make enough money to buy a suit of banded easily. Banded isn't some kind of magical armor that makes tanking effortless; I'd understand if you got a mosscovered twig and wanted to roll a monk, or something, but a suit of banded is not going to make a huge difference for any character, and is super easy to buy anyway.
2) The 40% EXP penalty doesn't matter nearly as much in a group. If you're grouped with four other people of equal level who have no EXP penalty, your 40% penalty effectively becomes an 8% penalty because the penalty is "spread" to the entire group. So if you are hell-bent on making a tank, don't let the penalty be what stops you from making a paladin or shadowknight. Both classes have excellent aggro abilities and are way less boring than warriors, who have all of four buttons to push.
3) As for the actual question: warriors do have trouble holding aggro, it's true. Taunt doesn't work on even-or-higher con mobs, period. On blue cons, it will often work, but it only puts your threat at 1 point above the previous maximum, so all that character has to do is hit the monster again and it's no longer on you if you're doing less DPS. It also doesn't work on mobs over level 50, if I recall correctly. Your weapon damage on its own won't be enough to hold aggro since you may have a monk, rogue, or ranger doing greater weapon damage. That leaves you with weapon procs, which happen randomly and still aren't an end-all-be-all for aggro holding. This isn't to say that warriors can't tank, but they're certainly at a disadvantage compared to knights in terms of holding aggro. Your group will have to take certain precautions for you, like waiting a while to let you grow aggro before they start attacking the mob, using roots and the like, and so on.
Basically, warrior tanking is definitely doable, but it requires good proc weapons and a careful group, and it's boring because you have almost no active skills at your disposal. Knights have a much easier time with aggro and more stuff they can do. But you shouldn't be making a character just because you got a suit of banded, period; you should think about whether you'd actually enjoy playing the character, and whether you'd maybe prefer to first level up your main (whatever it is) and get some plat so you can then twink out your tank alt (this would be especially beneficial for a warrior).
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