If you knew how to use BK, you wouldn't say that. It turns Ragna's rushdown up quite a few notches. With Blood Kain, you can turn 6D into a VERY good Air Meaty(All Leading to 4000+ Easily), Throw into Massive damage with a beautiful wake up tech trap(Do a few 5D cancels and make the last hit something easily techable after connection such as 5B or 6A. Then Revolver to a Meaty In Air Unblockable without barrier. They will be so close, it can and should lead to mix up if block'd. ), Mess with people's heads with Death Spike, (Take a look at what it does to barrier or Guard Libra. Blocking one is about 2 bars on the Guard Libra If I remember correctly. And its safe VS counters. ) Keep People on the def. side of things, Zone better(You could go air to air with Hakumen if you want now. j.D can be out of Haku's j.C and several of his special inputs simply because its taking up half the screen. Oh, and you can jump cancel it to now flat out dodge counters after the hitbox touches the Counter if your spacing is correct), Rake in Tension on block or hit and chip people out like crazy. A block string with BK and a Rapid should be a guard break. Or end with them having no barrier. One of the other. With the tension that you get from pullin' off basic things in BK, its not to hard to see.
Usually, BK in Desperation is a sad attempt to try and pull things back together. That "Last 50 Heat" vs most chars would be better use'd as a long blockstring into Hell/GH to Rapid than grasping at your right hand. Using it in a safer, aggressive way on the other hand. Much better Idea.