In fact, Twitter is discouraging developers from re-creating that same Twitter experience in different clothing (Tweetbot, etc.) and encouraging them to build apps into Twitter. That turns Twitter the soapbox into Twitter, Inc. the platform.
Gizmodo compares this to Facebook.
Here's what I think when I hear about "apps" and "consistency of experience": Twitter is trying to change their service from a place where users come to create content into a place where users come to consume content.
The reason is simple: content creators generally do hit-and-runs -- they take up their timeslice on the community soap box then move along. Consumers will stick around longer and thus be more exposable to advertising, which is good for Twitter in the long run.
This is a hard trick to succeed at -- take a large service with an established following and successfully carry forward both your service reputation and a critical mass of the audience through a complete paradigm shift.
Offhand I can't think of a company that's succeeded at that. Apple, maybe, although what they've done is more using their previous line of business (boutique PCs) as a reputation boost to invent totally new businesses (smart phones and tablets), and growing those new businesses like crazy such that they eclipse the original.
