What are the true benefits of ExpandoObject?

Since I wrote the MSDN article you are referring to, I guess I have to answer this one.

First, I anticipated this question and that's why I wrote a blog post that shows a more or less real use case for ExpandoObject: Dynamic in C# 4.0: Introducing the ExpandoObject.

Shortly, ExpandoObject can help you create complex hierarchical objects. For example, imagine that you have a dictionary within a dictionary:

Dictionary<String, object> dict = new Dictionary<string, object>();
Dictionary<String, object> address = new Dictionary<string,object>();
dict["Address"] = address;
address["State"] = "WA";
Console.WriteLine(((Dictionary<string,object>)dict["Address"])["State"]);

The deeper the hierarchy, the uglier the code. With ExpandoObject, it stays elegant and readable.

dynamic expando = new ExpandoObject();
expando.Address = new ExpandoObject();
expando.Address.State = "WA";
Console.WriteLine(expando.Address.State);

Second, as was already pointed out, ExpandoObject implements INotifyPropertyChanged interface which gives you more control over properties than a dictionary.

Finally, you can add events to ExpandoObject like here:

class Program
{
   static void Main(string[] args)
   {
       dynamic d = new ExpandoObject();

       // Initialize the event to null (meaning no handlers)
       d.MyEvent = null;

       // Add some handlers
       d.MyEvent += new EventHandler(OnMyEvent);
       d.MyEvent += new EventHandler(OnMyEvent2);

       // Fire the event
       EventHandler e = d.MyEvent;

       e?.Invoke(d, new EventArgs());
   }

   static void OnMyEvent(object sender, EventArgs e)
   {
       Console.WriteLine("OnMyEvent fired by: {0}", sender);
   }

   static void OnMyEvent2(object sender, EventArgs e)
   {
       Console.WriteLine("OnMyEvent2 fired by: {0}", sender);
   }
}

Also, keep in mind that nothing is preventing you from accepting event arguments in a dynamic way. In other words, instead of using EventHandler, you can use EventHandler<dynamic> which would cause the second argument of the handler to be dynamic.

One advantage is for binding scenarios. Data grids and property grids will pick up the dynamic properties via the TypeDescriptor system. In addition, WPF data binding will understand dynamic properties, so WPF controls can bind to an ExpandoObject more readily than a dictionary.

Interoperability with dynamic languages, which will be expecting DLR properties rather than dictionary entries, may also be a consideration in some scenarios.