The number one thing I dislike about Flash Events is that I can't send arbitrary data with the base event class. Sure I could've written a PureMVCEvent subclass with an object as a payload, but internally to PureMVC, I wanted a publish/subscribe system, the beauty of which is that the subscriber need not have a reference to the publisher in order to receive the notification. (i.e. thingIHaveARefTo.addEventListner()). Also, Flash defines the Event class, not ActionScript and by this juncture I was already sure I wanted to remain platform neutral, because AS3 was slated to become the next JavaScript at the time (via Mozilla's Tamarin project).
The notification system as implemented in PureMVC complements the event system in Flash, it's not meant to be a replacement for events. It operates in a fundamentally different way. For a detailed discussion of Events vs. Notifications, see page 13 of the PureMVC Best Practices document:
http://puremvc.org/component/option,com_wrapper/Itemid,174/-=Cliff>