Use Unleash if your MP is limited and you need to establish aggro badly. Unleash is cheaper than AD but produces the same aggro, at the cost of DPS. If your MP is fine there's little point to using Unleash.

Also as mentioned, try not to DA AD unless you are possibly going to die without it, or the pull is so large that Blood Price is giving you an excess of MP. Very, very expensive for relatively small healing, and DA DP will be a much better use of all that MP cost.