From Forum Nokia Wiki

ECOM ( Epoc Component Object Model ) and Polymorphic DLL are used for extending functionality at runtime, Prior to ECOM, Polymorphic Dll's were used for this purpose, there are some advantages of using ECOM over Polymorphic Dll's which are discussed below:

Common advantages which applies to both ECOM and Polymorphic Dll

1) Just In Time, i.e the DLL could be loaded as and when needed, which also avoids applications from using up memory.

2) No Recompilation, i.e Modification of the DLL do not require recompilation of code that uses it.

Advantages of using ECOM over Polymorphic Dll's

1) First and foremost is writing an ECOM Plugin is far more simple than writing a polymorphic dll.

2) In ECOM, an object interface can have more than one implementation.

3) The ECom framework provides a service to instantiate, resolve, and destroy plug-in instances opposed to polymorphic dll's where accessing objects in polymorphic DLLs requires knowledge of the DLL's location.