Hopefully, you have noticed, that a WorkItem is activated while one of its SmartParts respectively controls is entered?
It is independent to which of the WorkItem's collections (Item- or SmartPartCollection)  the control was added, it only has to be derived from System.Windows.Forms.Control.
This works due to the ControlActivationService of the CompositeUI.Winforms Library. If the control is entered the ControlActivationService activates the containing WorkItem. The SimpleWorkItemActivationService ensures that only one WorkItem is active.
Paulo Morgado shows us how to replace the SimpleWorkItemActivationService by an hierarchical ActivationService.

Go to the p&p Smart Client Guidance Community Site, download the current release and start playing with the samples out of the Smart Client Contrib package. They rewrote the original BankTeller sample. By comparing the original BankTeller sample with the one out of the SCSFContrib you'll see the advantages of a WorkItemController over a typed WorkItem.

The samples just show you what you can do with CAB, but to start developing your own app you need a deeper understanding of its mechanisms.
Therefore, I would propose to read the introduction of Rich Newman - it is the best I ever read. To me, it was most important to get a comprehension of the WorkItem and how it is used by SCSF (WorkItemController). Don't reduce it to a representation of an "use case", it is much more.

To build an application with CAB rely on Workspaces (CAB), the WorkItemController (SCSF), SmartParts (CAB), Services (CAB), Events(CAB), Commands(CAB) and Actions (SCSF). Don't use UIExtensionSites and the WorkItem.State.