This would be a very useful addition to the API. You could use it in an app that automatically deletes your bad tweets to reduce the number of followers that unsubscribe.
For example, you could specify that if two followers unsubscribe soon after you post a tweet, then delete that tweet. You could even take it further by specifying conditions on those followers (e.g., only count those with with at least 1000 followers). Any chance an efficient way to get recent unsubscribers would be added to the API?
