Is Nightfall useless (4% seems like a small number)?
Definately not. Those who have nightfall praise it as a very useful talent, and here’s the math to back it up. The probability of an event happening is equal to 1 minus the probability of it not happening. In this case we can find out the probability of having nightfall process once in the duration of Corruption or Drain Life. There are 6 ticks in a corruption cast, and 5 in a Drain Life (nightfall has a chance to process per tick), so the chance of getting nightfall to process in a single drain life cast is 1-(0.96×0.96×0.96×0.96×0.96) or 18.5% and for Corruption it is 21.7%. This means you should expect nightfall to process once in every 4-6 castings of these spells.
