A Short History of Disease
Unholy Blight has seen a lot of changes over the course of Beta. These changes fall into three basic categories: The power of the effect, the duration of the effect, and the utility of the effect.
Unholy Blight's basic function is to create an AE damage effect around the user that pulses and does damage within about a 10 yard radius. That's about the only way it's stayed consistent over the Beta, however. Sometimes, its duration and cooldown have been tweaked so that it can't be up all the times. Sometimes it applied 1 disease, sometimes 2, sometimes none. Its damage has been everywhere on the scale from nearly non-existent to incredibly powerful.
At its most powerful, it not only did a very large amount of damage, but it applied a fourth disease to go with the pre-existing Blood Plague, Frost Fever, and Ebon Plague diseases, causing all of the Death Knights various strikes to scale even higher thanks to the new disease. In addition, it applied the Blood Plague disease, freeing up an Unholy Rune for use for an extra Scourge Strike or Death Strike.
Unfortunately, the devs decided a month or so ago that Unholy was just too powerful. Part of the reason for this power was the 2 extra diseases, Ebon Plague and Unholy Blight, that Unholy Death Knights had over the other two specs. Since Unholy Blight was the only source of one of these diseases, and a convenient Runic Power-based source of another, it was the prime target for nerfing.
Thus, we have today's version: It no longer causes any diseases at all. Unholy Death Knights are once again limited to 3 diseases (which is still 1 more than Blood and Frost Death Knights), and they still have to sacrifice an Unholy Rune and maybe the ocassional Blood Rune to keep Blood Plague up. There's also some concern that Unholy Blight's damage is pretty low, to the point where many are arguing that they'd rather spend the 60 runic power it takes to use to get off a Death Coil or a Rune Strike, or least to summon a Gargoyle.
Unholy Blight: Problems and Solutions
There's 2 solid factions right now in the Unholy speccing camp. One believes that despite the nerfs, Unholy Blight is still okay, doing some nice solid damage to multiple mobs. Other believe that it is a very poor use of a 51 point talent slot, and would rather limit their spending to 50 points in Unholy, going for Mark of Blood in the Blood Tree, or even filling out Improved Icy Talons in Frost for maximum buff bot power.
The pro-Unholy Blight camp believes it still adds adequate damage, especially for AE battles. In fact, they say, going into Frost especially makes you give up far too many important Unholy DPS talents, especially Rage of Rivendare.
There is one other reason to consider passing up Unholy Blight though: Death Knight AE has been sort of buffed as it is. Pestilence is now good on unlimited targets, and Death and Decay is at a pretty decent place as well. In addition, Blood Boil no longer consumes diseases as it once did. There fore, you can actually do a very nice damage rotation that involves using Pestilence to spread your diseases and Blood Boil to erupt them, causing plenty of AE damage, especially when Death and Decay itself is put into the mix.
As to who's correct, I'd say it may depend on your playstyle. If you prefer to grind against single targets, Unholy Still has a lot to offer, but talents like Unholy Blight are really focused more on the AE side of things. For single talents, that extra Runic power really is probably better spent on a Gargoyle summoning or a Death Coil. If you like taking on multiple monsters at once, Unholy Blight is probably a very good place to spend your talent points and Runic Power, providing one more tool for taking them down fast, and if you're the tank, keeping them on you instead of the healer.
There is one final problem a lot of people have been putting forth is that it just didn't feel like a fun talent anymore, since it does nothing more than a simply point blank AoE with no special effects or even a flashy animation. Even with other low-impact 51 point skills like Titan's Grip, you at least get the feel of wielding two massive swords at once and everything.
Some of these people suggest adding another effect to Unholy Blight to make it feel fun again. Among the suggestions are turning into a Locust Swarm effect which returns some measure of health to the caster, or having it spawn carrion beetle pets, both ideas being taken from the Crypt Lord hero of Warcraft III. Others want it to get a Mortal Strike-like component like Anetheron's Carrion Swarm. Certainly, both ideas would definitely add to the fun factor of the skill, but there's also some concern if they'd make it overpowered. After all, health draining is generally considered a Blood Tree mechanic, and the devs have also said that they don't want to hand out Mortal Strike willy nilly as the answer to everyone's problems.
The counter argument from some is that Unholy Blight's damage is so low right now that it can take an extra effect without straying into overpowered territory. Right now, it may be that the latest iteration of Unholy Blight just needs some more testing until we can figure out where it really falls. Maybe it'll finally be in some form that most people can deal with by the time we get to patch 3.2.
Using Unholy Blight today
In the meantime, if you decide to spec Unholy Blight, a good AE grinding rotation that used it would probably look something like this: Once you've gathered a group of mobs or found a group you like, get Frost Fever and Blood Plague up on one of them as soon as you can. A quick Pestilence should then spread the disease to them. At that point, you may want to use a Blood Boil to get them all nice and bothered. From there, it's a matter of getting your runes refreshed to use Death and Decay, and activating Unholy Blight as soon as you have the extra runic power for it, then using your runes to refresh diseases (Tab to a new target and use pestilence on them to refresh the diseases on your main target). If you can conserve your health enough from battle to battle, you may even be able to chain pull new mobs, especially if you managed to pick up Butchery and Vendetta in your build, or if you've done a bit of reputation grinding with the Oracles of Sholozar Basin.
As to Unholy Blight's final usefulness, this is another thing where we're on the bleeding edge of a new class. It's tough to say how things will really shape up until we've had a chance to be at level 80 for a while on live servers, fighting things out, collecting data, and seeing where Blizzard's balance philosophy for Death Knights has been right and wrong. Of course, even with only a few weeks left until we see Wrath, you never know what might change. We'll keep you posted.