Since I work surrounded by lawyers, but I am not one myself, everything here is In My Humble Opinion, nothing more.
I'm most familiar with AUPs at academic institutions, since that's where I work. However, in researching college and university AUP documents, I've also read some Law Review articles about AUPs for business. Almost all of them say how you intend to use the AUP, and what it's purpose is, govern how useful it is as a legal document. Most go on to say that in a business, the most useful AUP is one that protects the business first, period.
That being said, I can see two uses for such a document:
1) if the AUP is a social contract (ie best behavior, acting like a responsible net citizen, not spamming others, etc.) then it is more a set of guidelines than policy. Nice to have, but would be difficult to enforce as a legal document. In many cases, I've seen this kind of document refer back to a legally enforcable document to have some power over users and management (ie, an AUP that refers to the parts of the union or employment contract stating that people can be fired for breaking the law on the job, and reminding folks that illegally sharing copyrighted material is breaking the law to discourage file sharing, etc.) In this case, the AUP points to the legal documents that would be used to enforce that policy, so you don't "reinvent the wheel." This kind of AUP is really pointing to behaviors and actions that are disallowed as an interpretation of those pre-existing policies and agreements (ie, the existing HR policies, in the context of information technology in use in your office.)
2) if the AUP is meant to be a legally enforcable part of company policy, then legal review is needed for many reasons. First and foremost, you need to make sure the AUP doesn't contradict any other legal policy or contract already in place. If the AUP allows something that the emplyment contract disallows, and you dicipline an employee for it, you may have quite a fight making it stick. All the other reasons for a legal review mentioned in the preceding posts should be enough reason to have a lawyer review it IF you plan on educating users AND enforcing the policy strictly. A legal review of such an agreement protects management and employees alike.
That's just my two cents. My view is somewhat skewed, as I've done a little research in the field. If you'd like to read a critical examination of academic AUPs, check out my paper at:
http://www.giac.org/certified_professionals/listing/gsec_100_4118.php
The first of my footnoted references is a Law Journal article I found with some good practical advice about AUPs for businesses. I hope this helps.