This threading model is not recommended for components to be called by ASP pages. Single-threaded objects are not suited for applications that need to accommodate a large volume of users simultaneously. Using these objects can result in a deadlock when multiple users try to access the object simultaneously. All calls to Single-threaded objects are serialized through one thread, the COM Main thread. Calls to these objects from other threads are marshaled to this thread. Only one thread at a time can enter a set of Single-threaded objects, and it must be the same thread each time.
Finally, Single-threaded objects work in the context of the SYSTEM account and have all permissions associated with the SYSTEM account. In most cases, this is not desirable for Web sites.