I would agree that iteration through thought is not necessarily a useful constraint on a medium by itself. In the case of twitter, however, it is merely a product of the 140-character limit for each message. If, during a stand-up debate, your opponent was allowed to interject after every sentence, you would get much the same. That sounds incredibly annoying and counter-productive at first, but the form of the debate would adapt over time.

The advantage of developing a thought in tandem with somebody is that the compromise happens more naturally. You rarely need to 'concede' a point or prove somebody wrong, since you can influence the way the point is made in the first place, pointing out flaws in their reasoning or facts as they happen. Conversely, the other can do that for your reasoning. The result is that, rather than adjust your thinking in a few big steps, you adjust it in minor ways with every sentence. It takes time and practice to get used to this, but it can be very productive and enlightening.

On the other hand, I would never avoid long-form debate either. Both have advantages and disadvantages, just as both have mediums where they work and mediums where they don't. Twitter isn't conducive to classical debate, just as hackernews isn't conducive to the iterative thought development style.