2) There is no one-one mapping between work items and threads. Work queues and the threadpool try to manage the number of threads based on the workload / CPU availability. For example, the Vista threadpool tries to keep # CPU threads running, but will throttle threads back when it detects this number has been exceeded. Additional threads are created as needed in this range; the excess work items accumulate and are serviced by threads as they become available.
3) It depends on your application. If it is a piece of code you just want to execute asynchronously, threadpool provides an efficient and simple means of accomplishing this. It does, however, introduce additional overhead. In other cases, you may need a dedicated thread for a task whose operation / life cycle you may need finer grained control over.