Mobile code is slowly gaining acceptance but it is still not clear where it is really useful. If not used judiciously it may incur greater complexity of programming and degradation of performances. The goal of this paper is to show that mobile code is particularly well suited as a glue for the composition of immobile services, where flexibility and extensibility are necessary. To support our claim we describe two services and one application that have been programmed with mobile code in the context of active networking. We study the impact on the flexibility, complexity and performances of the resulting systems. We observe positive effects on flexibility and complexity and acceptable performance penalties.