In 3.1 Pestilence does zero damage. That said, even now Pestilence is not competitive damage except MAYBE as Blood where it may compare to BB, but will not be worth more damage than HS unless you have 5-6+ targets to hit, and still it is small damage spread as opposed to high damage focused, which is a judgment call for the situation.
As unholy my Pest currently hits for about 400, and my BB for about 900-1000. For Frost neither is particularly potent.
Also, LOVING the new Glyph of Disease. It refreshes your diseases on the main target when you cast Pest, thus allowing one cast to refresh everything's diseases at once, no need for two casts any more.