I will let others answer with their individual experiences. If you already have a bandwidth shaper at your permimeter, you will only need bandwidth contracts if you perceive that users will be accessing high-bandwidth internal resources and you would want to make that fair.
Others who do not do shaping at the perimeter and would want to put a different class of service on say, guests and no bandwidth contracts for their regular users can certainly use this feature.
In Amigopod, there is a feature where a guest can sign up semi-anonymously for low-bandwidth internet, but still have an authorization email sent to someone within the organization. When the person in the organization gets the email and approves the request, Amigopod dynamically changes the role and perhaps expiry date of that individual, and they get more bandwidth, as a result. Employees at a company use this constantly to offload the task of authorizing contractors to the contractor's sponsor.