This ability lowers a bosses attack speed and is by far the biggest damage reducing debuff that can be on a boss.
The instinct is to say that you should have this up on every boss 100% of the time but there are many cases when this is not true. If a boss does not hit that hard and your healers are bored, you should not be using thunderclap. If the boss attacks more often, then you can be hit more for increased rage (and by extension, threat).
Wrong, its quite the contrary, if the boss hits hard then please never leave defensive stance to use thunderclap. let one of the DPS warriors or off tank put the debuff on... and if the doesnt hit hard, you can safely thunderclap.