so you have 7 shadows in approximates every 37second [(30sec+45sec)/2] because they don't share recast timer. That is 1 shadow per 5,28seconds. My point still stands.
This doesn't have anything to do with tanking abilities, Utsusemi was broken on any job. It boosts anyone's survival ability by huge margin. It was used in every hard solo attempt, not only in tanking.