Causation theory is a complex science and involves philosophy and the social sciences. Causation is rarely, if ever, a black-or-white issue. The first to appreciate this was Robert Koch who discovered the cause of TB; in fact he had to formulate his postulates to convince his peer-group that he had found the cause of TB. Even then it took Robert Koch more than a decade to convince the scientific community of the significance of his findings.
The issue of causation theory has come up recently in relation to our posts on CCSVI. If you are interested in reading more about causation and causation theory, specifically in relation to MS, I would recommend the following articles as a starter:
Giovannoni G, et al. Infectious causes of multiple sclerosis. Lancet Neurol. 2006 Oct;5(10):887-94.
Causation is not the same as association. But I think you are aware of that. A given symptom is often associated with many different diseases/conditions. Treatment of the symptom may bring relief to the patient. Does it matter whether or not the disease has been "cured"? Often not to the patient. If the disease is cured, but the cure was worse than the disease is the patient better off? Your medical opinion, based on expectations, might conclude YES, they are better off. The patient's opinion, based on quality of life, might be NO. Godspeed in finding the cause of MS, but meanwhile allow MS suffers to make their own choices as to how they choose to live the one life they have.