sorry to take this "back in time" a little - but I just wanted to share with you my own method of considering the value of "Nightfall".
Firstly though I have to make a point which most probably already know - and that is sitations where mana is not constrained (i.e. you are free to life tap or dark pact as much as you want), the appropriate method of deciding which is the best spell to cast (at any one instant) is to calculate it's "Damage pet Cast Time". Any class has to decide what it's critical resource is - and then to compare everything against utilising that resouce.
So having covered that, what I do is to take any spell - calculate it's "DpCT" and also factor into this the amount of time needed to lifetap that spent mana back. This is to calculate the sustained dps for a boss fight. This ofc assumes that you start and finish a boss fight on the same mana - which obviously isn't the case - but it's then trivial to factor that in after calculating the sustained dps.
So here's how I would calculate the "DpCT" of a plain shadowbolt. For simplicity I'm going to assume 100% hit rate and 0% crit, +1000 spell damage, no improved lifetap.
1 Shadowbolt - Cast time 2.5 seconds, Damage:1429 (average), Mana Cost:420
0.4 Lifetap - Cast time 0.6 seconds, Mana Cost: -420
So the sustained dpct of shadowbolt is 1429 over 3.1 seconds = 461 dps.
Now - with Nightfall - the only change you need to make to this calculation is the cast time. A 4% chance of a proc - reduces the average shadowbolt cast time for 2.5 to 2.46 seconds.
So the sustained dpct of the new shadowbolt is 1429 over 3.06 - which is 467
Obviously - 1000 spell damage, 100% hit and 0% crit - are completely arbitrary values I used for simplicity - but you could easily modify what I have done to take real conditions into account.
As an affliction warlock I calculate all my spells this way. At anyone point in time that I am free to cast - the spell selection criteria is based on giving me the biggest "Damage per Cast Time".
Anyhow, I hope this helps anyone who was interested in the Discussion of the appropritate treatment to apply to Nightfall.