Catch all exception good or bad?

I've seen in multiple projects a kind of catch all exception to catch all unexpected exception so the app won't crash, i see this usually with :

AppDomain.CurrentDomain.UnhandledException += new UnhandledExceptionEventHandler(myUnexpectedExhandler);
Application.ThreadException += new System.Threading.ThreadExceptionEventHandler(threadExHandler);

Is this a good or bad practice.