“The discretion to hear disputes, even in the area of public law, must, however, be exercised with caution and appeals which are academic between the parties should not be heard unless there is good reason in the public interest for doing so as for example (but only by way of example) where a discrete point of statutory construction which does not involve detailed consideration of the facts, and where large number of similar cases exist or are anticipated so that the issue will most likely need to be resolved in the near future”