As Twitter has grown, so has the Twitter app industry. To begin with, you could only use Twitter on their website. Now there are literally thousands and thousands of ways to use Twitter. Twitdom holds a directory of over 2000 Twitter applications, and this doesn't include websites that post, display, or interface with Twitter, or applications not registered with the site.
Today I read the following article in the Ars Technica blog on my Kindle. It raises some concerns.
On the desktop, I have tried many Twitter clients, some web-based, some traditional applications. Of these, Seesmic Desktop is the one I've found most useful. The abilities to use it with other services besides Twitter is very nice, and the support for multiple accounts is the main draw for me. But most of the time, I use the normal Twitter website when I'm on the computer. With Chrome plug-ins to allow URL shortening right on the Twitter website, and using a separate website like twitpic for pictures that has the ability to post to Twitter, the website works pretty good. But other people have other needs.
On a mobile phone, the Twitter website isn't as useful. Twitter's preference seems to be for people to use their application or to use text messages. If you use a supported phone, the application is an option, but there's phones that only support third party applications. If support for third party apps is removed, this will only leave text messages. Some people love doing Twitter that way, while others don't.
As I said, different people have different needs. It is impossible for one client or application to do everything that everyone wants or needs, and to do it the way each person wants. While Twitter has come a long way with their clients, third party applications are necessary to meet this wide range of wants and needs. Part of Twitter's current popularity is all the things you can do with it because of this wide range of applications. If Twitter limits desktops and mobile devices to only their client, many users will no longer be able to do what they use Twitter for. Only time will tell it Twitter has reached a critical mass where they can eliminate people's ability to do things they don't support and still maintain the momentum they currently have.