Self-determination is crucial in social work. It is a very hard profession. You need to be determined or you are never going to make it. You have to have a great deal of personal strength. You have to know you are doing good, and handle the emotional nature of the work.
I assume you're talking about self-determination for the client here. If so, then I would agree that it is very important for the field of social work.
If clients do not have self-determination, then the social worker becomes more of a dictator than a helper. If the client has no self-determination, the social worker is telling the client what to do. This would make it very likely that the social worker would try to impose his or her values on the client instead of working with the client to improve the client's life on the client's own terms.