. You want a stateless services
. Your service hold intensive resources like connection object and huge memory objects.
. Scalability is a prime requirement. You would like to have scale out architecture.
. Your WCF functions are called in a single threaded model.
. You want to maintain states between WCF calls.
. You want ok with a Scale up architecture.
. Light resource references
. You want share global data through your WCF service.
. Scalability is not a concern.