Is there any reason to make POCOs into Model objects?

Validation is the main reason not to bind directly to a POCO. In addition, if the POCO doesn't already implement INotifyPropertyChanged and other required interfaces, the experience working with the object on the WPF side may be less desirable, and implementing a ViewModel to wrap this makes sense.

Providing a ViewModel to wrap your POCO allows you to encapsulate the logic into ICommand implementations as well as implement required interfaces cleanly.

I disagree only slightly with Reed (an unusual circumstance to be sure). I would NOT implement a ViewModel to wrap the POCO. I would implement a Model class to wrap the POCO and expose the Models to the ViewModel via a Service layer.

The ViewModel's primary job is to appropriately present Model data to the View and react to its requests. The architecture I'm working on for this looks like so:

• 1 ViewModel for each View
• The ViewModel calls a Data Service layer object to retrieve Model instances (not to be confused with a WCF service)
• The Data Service layer issues the appropriate CRUD requests to the backend (this uses WCF, RIA, or RESTful Services for Silverlight but could be ADO.NET or EF directly for WPF).
• The Data Service uses the returned POCOs to create Model objects.
• Model objects wrap the POCO object and implement INotifyPropertyChanged. Model objects enforce business rules.

I'm still working through the details but I will be publishing something more concrete in the near future.

My Models accept a WCF object which exposes those properties which I wish to use in my ViewModel. I can then also extend the object as needed. My properties point to the WCF object's property and when I have to send the object back to the WCF service, I don't have to do any more work. The models inherit INotifyPropertyChanged and INotifyDataErrorInfo which the DTOs (mentioned here as POCOs) will not have. Your business logic / validaton exists in your Silverlight application and not in your WCF Service.

The View binds to the ViewModel which has a Model (or an observable collection of Models). The Models have a WFCObject which is a DTO (mentioned here as POCO). I use my ViewModel to communicate with the service, MVVM Light has the models communicate with the service / provider - which I don't like.