I don't know if this is strictly related, but I use a ProxyHelper class which exposes Proxy instances for me as a convenience.  Then I can reduce:

var proxy:SomeProxy = facade.retrieveProxy(SomeProxy.NAME) as SomeProxy;

to

var proxy:SomeProxy = ProxyHelper.getSomeProxy();

I would hazard a guess and say that this is for the same reason that PureMVC uses Notifications rather than Events - to remain completely platform agnostic.  The Event mechanism is a Flash/MX'ism and depending on it ties PureMVC down to a specific implementation.