I agree that nobody should be forced to wait to pull a mob. I simply consider it common courtesy, and those who are not willing to extend it to be rude and selfish. Pulling a mob when it's just you and maybe your group when there are several people shouting that they are on the way and desperately trying to assemble a party while running across the map is simply saying "no, it's MY mark and only MY party will get the credit for it because it's all MINE."