When I tested NP Crobat (and I retested it a few days ago) I found the set to be unappealing for mainly the following reasons:
a) Crobat's SpA is too low, and even after a Nasty Plot it's not that strong and continues missing a lot of KOs and 2HKOs on several Pokemon.
b) The lack of Roost is pretty serious. With Life Orb, SR, and the fact that sometimes you'll only be able to set up Nasty Plot at the same time Crobat's taking an attack (for instance: Celebi's HP Fire / Grass Knot; a Choiced Pokemon locked into a non-effective attack; or some other random stuff) reduce Crobat's HP considerably fast, to the point that in occasions you'll be better off attacking rather than using Nasty Plot to boost your mediocre Special Attack. Evidently, one can choose to use Roost instead of Hidden Power, but that reduces coverage a lot IMO.
c) The combination of Sludge Bomb / Heat Wave / HP Fighting is very decent, I must admit, but there are still a large number of Pokemon who have no problems switching in on said moves repertoire. Swampert, Vaporeon, Suicune, Gyarados, Tentacruel (bulky Water-types overall) are a major inconvenience for Crobat, while Heatran can take a hit (or various if it's bulky) and eliminate the bird as well -- Heatran's problem can be remedied by using HP Ground instead, though this would make Tyranitar be a nightmare). Hippowdon, the aforementioned Tyranitar, Flygon, Snorlax, Blissey are some others to add to the list of potential counters.
In general, too weak, inefficient. I even used the set in tandem with Spikes to see how good the extra damage could be, but that really didn't solve any problem. I don't approve the set, though if any other QCer would like to go out and try it out as well, feel free to do so, that'd be excellent.