Really bad observation, the rules of public tank are:
1)hook anyone if you can kill the victim in 5 sec or when next to multiple allies to deal fatal amount of damage
2)weak team? no one to shield? Tesla farming that is
3)your team is bad? time for shield practice that is! No need to leave cause of -50%
4)Shield Slam only in 2+ enemies at a time, zerks and frosts are exeptions
5)never use battle rage with enemy zerk on stack around but feel free when enemies are frozen