Pamela Samuelson has self-archived "Justifications for Copyright Limitations & Exceptions."
Here's an excerpt:
The essay traces the historical development of L&Es in U.S. copyright law. . . .
The essay then considers nine justifications for the existence of these L&Es. One set promotes ongoing authorship. A second recognizes both authorial and broader public interests in dissemination of news, freedom of expression, and access to information. A third protects privacy, personal autonomy, and ownership interests of consumers. A fourth aims to fulfill certain cultural and social policy goals. A fifth enables public institutions, such as courts and legislatures, to function more effectively. A sixth fosters competition and ongoing innovation. A seventh exempts incidental uses lacking in economic significance. An eighth addresses market failure problems. A ninth encompasses L&Es adopted for politically expedient reasons.
It also discusses a tenth type of L&E, those designed to enable copyright law to be flexible and adaptable over time.
The essay concludes that the optimal policy for L&Es may well be to have specific exceptions for categories of justified uses that are relatively stable over time and for which predictability is more important than flexibility and to have an open-ended exception such as fair use to allow the law to adapt to new uses not contemplated by the legislature.