You are way overthinking this.... There are way more things to consider to value the worthiness of Blood Boil Spreading diseases to a new target instead of plague strike.. While plague strike is single target, blood boil (at the minimum) hits 2... There is also refreshing subsequent diseases of those other diseases on other targets other than the primary... Not to mention availability of Death runes prior to the target switching which makes it even difficult compare Blood Boil and Plague Strike
This is why calculating rune desyncing for targets more than 2 is not worth the pain.
Obviously you are taking roiling blood. Blood boil doesn't spread diseases, pestilence does.
Using Roiling Blood to refresh disease can lead to sub par (weak) diseases being spread if your not careful about it, considering blood boil is our main AoE attack. In 5.2, in AoE situation, your trinkets will proc, you'll hit plague strike (You'll always have that unholy rune) then Pestilence. All the mobs will have buffed disease and your free to Blod Boil to your hearts content. Roiling Blood is a DPS loss.
In an AoE situation, why would Roiling Blood be a dps loss? As long as you re-apply your diseases (when everything is up) to the closest target to you and then Blood Boil it, it should send all your super diseases to everything. Further Blood Boiling should keep the super diseases up regardless of whether stuff is up or not. Correct me if I'm wrong, but I believe that is how Pestilence works? It should send out the current state of the diseases to everything else rather than re-snapshot.