Because most DPS i get in my groups forget to LB or wait to start casting when the boss have like 1% HP left and the boss die before they even finish casting. Not, it's not required to beat the dungeon but sometimes it helps a lot when you get a group with undergeared players in fights like Ifrit HM (nails) and Haukke Manor HM (last add).

But asking people to LB is really that rude? I just say "LB" and nothing else, does people really get offended because of that?