Twitter is making it decreasingly attractive for 3rd party developers to bother creating applications for their platform:

[I]f you are building a Twitter client application that is accessing the home timeline, account settings or direct messages API endpoints (typically used by traditional client applications) or are using our User Streams product, you will need our permission if your application will require more than 100,000 individual user tokens.

In effect, this limits the total number of users of a 3rd party application to a maximum of 100,000.  If selling 100,000 copies of your application isn't profitable, you might be interested in backing

Read: Changes coming in Version 1.1 of the Twitter API | Twitter Developers