Warning: I've done no testing.
I was gonna save Cabal Therapy as secret tech, show up at the next Waterbury, and rape all the oath decks... but i sort've quit playing magic (except type 4 and limited, which also explains why I didn't do any testing) so I decided to just talk about it. I haven't seen a cabal therapy discussion (probably because I don't come on TMD as often as I did before) so if this has already been talked about, please close it down.
Back to topic...
(1) Planar Void (actually, I did see a thread on planar void i think... but I don't remember anything about it). Planar Void isn't really a silver bullet to every deck. It hurts them, but doesn't by itself shut them down completely. Planar Void is a reactive card, and therefore doesn't do what we want. If we are to use Planar Void, we need to take advantage of the slight "tempo" and "virtual card advantage" by playing aggressive. Cabal Therapy is much more aggressive.
(2) Cabal Therapy
"You tap your Forbidden Orchard to make one green, give me one token, and play Oath. I flashback therapy naming Force of Will. I play Seal of Cleansing."
With Oath running around rampart, Cabal Therapy should be seeing MUCH more play. But it doesn't. It's brother Duress is weaker in comparison IMO. They're both really good cards, but I think Therapy at the moment wins out. Therapy can hit things such as Goblin Welder, Lavamancer, Lackey, Etc. And assuming that Welder decks are still popular, you never wanna duress a Welder deck.
The problem is, there's no aggressive black deck that can use Cabal Therapy. Suicide black? Sui Black has some good things about... but it's just not the right deck still.
So I know I said I quit Magic... but Winter is coming back around and that's when I get back into Magic. Hehe.
I'm currently working on a few decks...
(1) Sui Black. I just might show up at Waterbury playing Legend Suicide just for fun.
(2) Extended B/R Goblins. With Aether Vial + Therapy. It works in extended, why not Type 1? Remember when I played FCG for a while?

(3) That U/W Aggro-Fish wannabe deck with standstill and aether vial. I'm going to throw black into it and make it "Mini-EBA"
Yeah.... cya guys at Waterbury.
edit: fixed a typo