Necrotic Plague seems rather underwhelming

[Sarkol]

Not really, since the longest duration is USUALLY, the one with the fewest stacks. It is possible to game for Unholy and Blood, but not so much for Frost when it will use HB has the spreader and applier. However, the average player, and even the top tier player on accident, will spread a weaker disease.

The situation where Unholy can make huge use of it is any close proximity add fight. Thinking the first set of adds on Garrosh. With Unholy Blight you would have a 15 stack up very fast, and when the adds come, you can pestilence and spread that, or just let it stack itself. As long as you maintain a 15 stacker on one mob, as long as it's in range of other adds, you'll have maximum damage bouncing around.

Shouldn't be THAT difficult to maintain it on one mob i guess.

[Aeriel]

The situation where Unholy can make huge use of it is any close proximity add fight. Thinking the first set of adds on Garrosh. With Unholy Blight you would have a 15 stack up very fast, and when the adds come, you can pestilence and spread that, or just let it stack itself. As long as you maintain a 15 stacker on one mob, as long as it's in range of other adds, you'll have maximum damage bouncing around.

Shouldn't be THAT difficult to maintain it on one mob i guess.

Still, it's not that OP considering that diseases are UH's primary AoE. Blood will gain more DPS/threat from Pest, and Frost's AoE is focused on HB. In an AoE situation, Frost might even be best beneficiary of NP as all of the diseases would have the same number of stacks (excluding primary target due to PS and FS+Plaguebringer) and duration, since NP will applied by HB and not Pest. UH would require a build-up period to maximize NP damage in AoE, which will help balance it. Blood will have the issue of different durations to balance its damage output.