Well, that's my dream. tent.io is nice, but it's not going to replace Twitter anytime soon, and I actively dislike app.net. But we're past the age when third party clients can be effectively blocked by technical means, and I want to see their return.
Plus, these cat-and-mouse games require a user base that is fairly quick to update; if Twitter blocks one avenue (and they will try desperately to do so if one of these clients gains a decent number of users), an update for the client is likely necessary. I would think the easier way would be for users to get their own API keys and secrets and enter them into the app.
[1]: https://twitter.com/tos, Section 8
Even if Tweetbot states something like "Twitter has limited the number of clients we can sell" they'll still think Tweetbot is stupid. But most of them won't think "I wonder if there is a way around this." And then head to some warez type solution.
Instead they'll look and see if there is an alternative, or more likely, just stick with Twitter's website.
I just don't buy the "This will drive people to pirated software" argument.
There is a subtle distinction here.
I'm addressing the underground marketplaces where Tweetbot-like applications already flourish. These apps, quite often, don't use any API at all but instead rely on hidden browser windows and net.webclient interfaces to simulate browser actions.
They'll just use the next most popular Twitter client for iOS, which is likely free or perhaps a buck or two.
Unless Twitter, and other widely adopted platform providers, find a balance between corporate interests and supporting users interacting with their services in the ways that they prefer to do so, we will see more and more people obtaining Tweet-bots and Friend-adders, from illegitimate sources which bypass API's altogether.