Interface Classes..Missed somthing? (NOT RESOLVED)

[Rdn]

As I have understood..
Interface classes are used inside COM components, inorder for the developer to maintain CallBack procedures, these callbacks are notifications sent between the client and the server.

Implementing interface classes requiers that the class contains only the definition of the methods and procedures, while the acting code is in a class contained in the client application.

And as I have understood, the most significant advantage using COM components, is the ease of update, that is, you update the server only in one place, rather than updating each client application.

Here rises the question:
In case of using Interface Classes, don't we loose that significant advantage?