The benefit is that your getter function will be bound to that event, so any time that event is dispatched, the data binding is updated.  In this example, it doesn’t really stand out that this is a great thing.  Let’s say that there is also a “get lastname” function that is similar to your firstname function, and add an updateNames function, so we have this…

[Bindable(event="MyEvent")]

public function set firstname(value:String):void

{

            _firstname = value;

            dispatchEvent( new FlexEvent('MyEvent') );

}

public function get firstname():String

{

            return _firstname;

}

private var _firstname:String;

[Bindable(event="MyEvent")]

public function set lastname(value:String):void

{

            _lastname = value;

            dispatchEvent( new FlexEvent('MyEvent') );

}

public function get lastname ():String

{

            return _lastname;

}

private var _lastname:String;

public function updateNames ( myUpdateObject : Object ):void

{

            _firstname = myUpdateObject.firstname;

            _lastname  = myUpdateObject.lastname

            dispatchEvent( new FlexEvent('MyEvent') );

}

The updateNames function dispatches a single event, which updates the bindings on both of your “getter” functions.   Now, the real benefit of “set” functions is that you can programmatically execute code when a value is updated.  Your names example doesn’t take full advantage of the capabilities.  The “set” function is better suited for when you have other functions that should be executed any time that your “set” is updated.

[Bindable(event="MyEvent")]

public function set firstname(value:String):void

{

            _firstname = value;

       //execute some code here

       doSomething();

       doSomethingElse();

       doAnotherThing();

            dispatchEvent( new FlexEvent('MyEvent') );

}

In this example, the data binding gets updated after the other functions have been executed.  This technique is VERY useful when developing flex components.  For instance, you can have a function on a custom component that extends a text box such as:

override public function set text( value : String ) : void

And you want other properties to be updated when text has been updated, so you setup your “set” function like this:

override public function set text( value : String ) : void

{

            this.updated = true;

            resetComponentState();

            super.text = value;

}

Hope that helps,

-Andy