Well, however, the 'killing the app manually' is useful while you're testing your app. We had a situation where the app is not the culprit but a middle-layer that authenticates through Facebook. As we've not yet taken care of the middle-layer failure, our iOS app was unable to figure out what to do but sent us in a Limbo.

Killing it manually help us 're-start' the app so we can use it without doing the 'un-install' and 'install' all over again.

Outlined in an article I wrote recently - http://brajeshwar.com/2011/kill-a-background-app-in-ios/