Well, a Storm "program" operates concurrently on many nodes at once. If an exception is thrown, you may want to log it and the stack trace, but where? If you write to a local log file, that data will be useless unless you run some sort of log shipping or log centralization (like with scribe or kafka or syslogng). But that's usually a pain in neck to setup and you can't run storm without already running a zookeeper cluster, so if you're lazy, you just log to zookeeper.
Everything is fine as long as exceptions are infrequent.
