BOTD on its own isn't that amazing, but when you get to 60 you get Geirskogul, which is. Also consider that you're trading a 150/180 potency opener (true thrust/impulse drive) for a 290 (200) potency fourth hit, then going to your next combo opener. Also, also consider that the fourth hit costs 60 TP for 290 potency, as opposed to 70 TP to start a new combo--it's not a lot in itself, but it adds up quickly in a fight when your GCD is down around 2.4s.