The most useful I've found cover is for when I'm running WP with my very geared BLM FC mate. He'll pop a double flare with raging strikes up and without quelling strikes, and it's more or less a sure thing that he'll get aggro on one or two things. He just knows I'll react quickly and cover him while I get the threat back. Additionally, it can be used in certain situations where you are off-tanking to pop a CD and cover the main tank when he/she is out of cooldowns or saving them for a more important moment, but it's very rare that you can get away with that because of cleave mechanics / other things you should be doing in the fight.