So Blood DKs have:
Originally Posted by Carighan
Anti Magic Shell
Ghoul / Deathpact
and Dancing Rune Weapon
Everything on that list above Dancing Rune Weapon isn't subject to RNG. You do have more hp, take less damage, or obtain more healing. DRW could do a lot for you, but because it's a chance, it could also do absolutely nothing.
I haven't done beta, but I am hard pressed to come up with a situation I were the developers intended for you to use all (4-7) cooldowns in 1.5 minutes. Because of this, unless a very specific situation demands it you could look at DRW as a threat / damage boost with a nice chance of increased parry. Rather then YAY Avoidance CD ... with some extra threat / damage.
It would be awesome with ERW, but I mean you could also just Rune Strike a tad less on one pack (if it's nearly dead anyway) and start off with DRW when you engage the next pull since You'll likely have mroe then enough RP and have most of your runes reset.