1.) The right not to be sued for thousands of dollars.
2.) The right to say you didn't lie, cheat, and steal your way through life, stepping on others for your own benefit.
3.) The right not to be labeled a criminal.
For most, that should be enough. However, if you're concerned solely with the material benefits for simply following the law:
1.) Vista will not lock down on you, limiting every activity you attempt to perform.
2.) You can download all non-critical updates and add-ons.
3.) The security of knowing another individual has not inserted malicious code into Windows.
4.) You can receive technical support from Microsoft.
5.) Your threads here in the Cnet forums will not automatically be locked for piracy.
P.S. For all responders, do not discuss the download or use of pirated copies of Vista.