Maybe there is some added value that I don't see. But, really, it looks a lot like Yelp, GetSatisfaction, or Ripoff Report: they create this big community focused on saying stuff about your app and it ends up costing you if you participate (in $$$) or if you don't (in customers).
Twitter already has an incredibly effective app-discovery mechanism: the "from AppName" that appears on every tweet on the website. And, that mechanism is 100% free (for now). I don't really see how oneforty improves upon that.
Twitter app development is kind of a no-mans-land right now. Any day, Twitter can change the cost model of developing apps for their platform. Recently, Twitter just made app development much more expensive (in manhours) by adding features that are fairly complicated to implement. In the future it seems likely they will ask for some direct financial consideration too--e.g. charging for use of their trademarks and/or charging for increased API rate limits. So there's a lot of risk from the cost side, because you don't know how much it is going to cost to develop & maintain the app.
Plus, there is the problem that 30% of 0 is zero while 30% of $1.00 is far from zero. These AppStore taxes hurt people who charge money for their apps and help the people who don't. In fact, the people who charge money end up subsidizing their own competition in a not-so-indirect way.
Now, maybe Oneforty has some plans to add value that is worth more than the value they are taking from app developers. But, reading through the material they present, I haven't seen it. I sincerely hope that they come up with a business model that is a net benefit to themselves and to app developers. But, until then, It seems less risky for (commercial) app developers to nicely ask Oneforty to pull their apps' listings from the site.