They're probably telling you to use LB when you are in a boss fight and its just about dead. As a DPS your LB seems to be the most useful for dungeons. As I tank I think I used LB twice in my entire time playing. As for the people, yeah it can be tough, people can be donkey's. I've had my fair share as a tank, but I don't let them get to me, most of th time I just ignore them, get the duty done and maybe the next group will be a decent one.