This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
I think this is a fairly FAQ. I would guess that the answer is that if Servlet didn't implement Serializable then it wouldn't be able to be serialized. It only needs to implement Serializable if it needs to be serialized. The question then becomes why would you need to serialize a Servlet. I don't know the answer to that and, perhaps, neither did the original developers. Maybe the decision was taken to implement Serializable in case some valid reason for serializing a Servlet materialized. After all, there are no obvious adverse effects of this implementation.