#1 Its a DPS loss usually, if not used at the start of a fight and even then there shouldnt be a reason for having to us it.
#2 If you are tanking properly this should never be an issue past the very early levels of the game. Tanks need next to no lead in time, and, barring special fight mechanics, should never be coming close to losing threat to the DPS.
TL;DR Stop trying to tank in DPS gear and using your DPS stance and this should never be a problem for you.