I actually have to disagree with those recommending swapping endurance for mainstat - there really is no point to it. I could buy it for a jugg/guardian due to force scream buff, but for other tanks having that extra cushion is useful for "oh ****" moments, like if the other tank just died and you have to tank both bosses at the same time (or tank TFB with force scream on you - the jugg tank in my guild has done it even in 16 man, buying us the time needed to res the other tank and get him back to work) or if you hit an enrage, either a hard enrage or a soft enrage triggered by mechanics such as low HP. This is especially true for shadows/assassins due to the buff to self healing - IMO it's inexcusable to make that trade with those classes, you already have the best threat of any tank and don't need to swap stuff around for that reason, and endurance actually imporves your mitigation - if you can pick it up somewhere without sacrificing mitigation you should. I even go for the B mods on my assassin for that exact reason.
Just because it may
be useful in those limited situations where the healers and/or the other tank are being incompetent, doesn't mean it's justified over the more optimized setup