You should be doing this anyway regardless of if you are one DK or 20... Keeping both diseases on your target, even at the cost of a GCD and spreading it is more of importance than a 1m+ CD on an ability... GCD > 1m CD. Also Rolling Blood refreshes the duration of your diseases, as well as spreading them to others who may join the battle in the meantime... that 1 Min CD is just too freaking long.
This is about doing the max potential DPS, the damage you are putting out the more a healer has to heal and the more cleave the better, this is one of the main reasons Frost DK is a god class in rated BGs as they provided the cleave damage that no other class can really keep up with.
This is the case with every class that has to deal with this, Ret Paladins suffer with crappy self healing, Spriest suffers from ****ty healing and their PW:S is effected by BF too, as is Feral healing, Ele Healing, Warlock Healing, Boomkin Healing... Do you care for me to go on? MOP turned a lot of classes around. The game took away all the survivability some classes had pre-Cata, to make up for this there is a lot of CC within the game.... Sadly Dk is one of the classes with the worst CC, and generally one of the weaker classes in general.
So you either accept the fact that DK is a pure glass cannon like it is, or you find some other class to mbox.