I've come across a number of argument-structuring tools in the past. I think doing this right is much harder than people give it credit for.
The core problem is that most of the action in a quality discussion doesn't actually consist of claims and counterclaims. For example, I went to Truthsift and the first claim I found on their randomized front page was "vaccines are safe" (http://truthsift.com/search_view?topic=Are-Vaccines-Safe-?&id=406&nid=4083). The response I wanted to make is that vaccine is too broad a category; a well-managed discussion would first clarify that the discussion was limited to the sort of FDA-approved vaccines one would normally be prescribed, not to random research chemicals, then break into sections about specific vaccines and vaccine additives, the trustworthiness of the FDA approval process, and so on. But Truthsift wouldn't let me say that, first because I'd have to somehow mash that into a pro-and-con structure, and second because I'd have to merge that into an incompatible structure that someone else set up. This is entirely representative of my experiences with argument-structuring sites: I show up, find an elaborate structure that partitions the question in a way I think is wrong, and bounce.
Arbital is currently better because it doesn't try to structure everything; it leaves space for all the hard-to-tag irregular stuff to happen in comment threads. I think there's a large valley in between comment threads and a structure that can incorporate all the irregular things that happen in discussions, and that TruthSift's mistake was that it tried to squeeze out the irregular stuff.