I don't know what it's like with the new patch. But in CM and Prae if you spam enough Flashes as a PLD in shield oath your hate is rock solid and all the mobs get aoed to death. There's almost no reason to mark in those speed run situations. Only when things get tricky you should mark. Or if you're doing something for the first time through.

You can usually tell what the tank is attacking by looking at the target of his target. Hotkey of T on default I believe.