You can see Delegate usage in several demos, including Cafe Townsend.1
I also used one just for fun in the DeploymentConfig utility. 2
But really, there's no need for it. It could be useful if you have more than one Proxy using a service. But really, it's use is promulgated by Cairngorm developers, who are making their calls from Commands, short lived objects.
Proxies are long-lived objects which can instantiate the service once and call it many times thereafter without need for setup.
In a Proxy's onRegister you instantiate the service and add listeners for all the result and fault methods. Remember there may be multiple service methods being called (on a RemoteObject) so you may have several handlers. So I don't like to implement IResponder, forcing me to have just one result and one fault handler. Just add listeners in onRegister and remove them in onRemove (if the Proxy will be removed before the program ends).
Cafe Townsend Demo source http://puremvc.org/pages/demos/AS3/Demo_AS3_Flex_CafeTownsend/srcview/1
Deployment Config Utility source http://svn.puremvc.org/Utility_AS3_Flex_DeploymentConfig/tags/DeploymentConfig_1_0/src-multicore/org/puremvc/as3/multicore/utilities/flex/config/model/