Registration: Username and password or email and password

During a registration process, what would the reasons be for using either username/password or email/password combinations to register the user?

• Is it a case of identification once in the system (it may be preferable to refer to a user by their username instead of their email)
• Simplicity - using email means the user doesn't have to create two
  memorable items?

What other reasons are there for choosing either option? Is there a relationship between the types of service?