The way this game works kind of determines that, unfortunately, there really is no point to the tank LB. It saves the healer a very small amount of MP, and that's pretty much the end of it outside of a very very specific handful of situations that are almost impossible to plan for. I've kind of wished LBs were individual for a long time now; that way the tank LB would be a helpful boon during a high damage phase rather than a griefing tool.