Putting aside that tanks tend to either have high value limit breaks (PLD, GNB), extremely high damage output if left untouched (DRK, GNB) or are actually squishier than you give them credit for (WAR, GNB), you need to kill a tank at some point anyways.

Efficient target prioritisation isn't "kill these roles / jobs in this order", it is capitalising on info like someone having recently used Guard / Purify and / or other cooldowns, as well as low MP so they have less heals.

Also it is better to attack what others are already attacking. Five pool noodles all hitting the same guy can still do something.