I think that if earlier tank LBs simply lasted longer they would be useful. It's not worth using most of the time when a tank's own raid-wide mitigation, Reprisal or healer shields achieves a similar effect, but if they would absorb damage over a long period of time it could be a significant enough advantage to use it in some fights. They could also be 80% and 160% shields instead that expire after 30 seconds or when fully absorbed.