Exactly. Imagine what Twitter could be with third-party developer support, and then ask yourself if that would be worth about $5 a month. Or just wait for half a year as stated.
But there's no way app.net is going to be Twitter without ads if you have to pay $50/year for it. It may be Super Small Niche Twitter, but it won't come close to being actual Twitter in numbers.
The problem is most people are not devs and don't care about twitter tightening up their API rules (which is entirely up to them), I understand and applaude the idea as a developer but I can't see it rivalling twitter.