Twitter is to identi.ca as an exclusive country club is to a public swimming pool. We might be denied access to Twitter’s platform for whatever reason, stated or unstated. This isn’t hypothetical, they actually put it in writing. However, they’ve said they’ll recind it. But then they could rescind the rescinding. It’s a lot like Apple’s policy with developers for the iPhone which might go something like: “We can’t let just anyone develop for this.”
identi.ca is different, like the Internet itself, no one gets to say whether or not you can develop for the platform. Sure one particluar instance of identi.ca might block your app, but they can’t all block it. (Not entirely true, by the way — just a lot less likely. Email, an open protocol, does effectively block some spammers from dumping mail through open relays. The real world sometimes forces across-the-board restrictions.)
To understand where we’re at, remember that software is a process, you can’t judge it by where it is today, you have start with that, and judge how it’s evolving. Twitter is struggling with finding a happy scaling place and a business model that sustains it after venture capital. As a corporation, they have incentives that an open source project doesn’t have. They’re more likely to pay attention to users’ needs than an open source project that’s more likely to tell you to fix it yourself. Though the lack of a business model has made it less likely that the company views its users as customers. They’ve been polite, even playful, but the service has been pretty awful.
I believe we need both. Single-party systems suck, I like (at a minimum) two parties. Everyone benefits from competition, users, developers, even the entities providing the service or product.