Why would I want to implement Externalizable instead of Serializable?

By implementing Externalizable yourself you can win performance at the cost of flexibility and extra code to maintain. If you implement Externalizable yourself you stream the data directly without the need for reflection which is used in the case of Serializable.

See http://developer.java.sun.com/developer/TechTips/2000/tt0425.html for an example.