You're right that correlation is not causation, and as you say there is really no way to carry out the experiments that are required to prove causation. The same problems would apply to "proving" anything relating to simple carbohydrates.

What we have with MS is some pretty strong epidemiological evidence, plus a plausible biochemical basis. It really amounts to what standard of proof you want, but it's good enough for me:

- MS is much more prevalent in inland Norway than on the coast, the primary difference between the two being that the inland diet is heavy in saturated fats from a dairy based diet (lots of milk, cheese and butter) whereas the coastal diet is heavy in fish (lots of omega-3). Lots of other less direct evidence but this is the one that sticks in my mind as far as saturated fat / omega-3 issue goes.
- Swank's longitudinal diet study indicated that long term progression for MS patients was greatly reduced (order of magnitude improvement) in those who could stick to a diet of less than 20g of saturated fat per day. This is a much larger effect than any of the medications that have ever been tested.
- omega-3 is the building block for a group of cytokines that suppress inflammation, which presumably includes inappropriate immune interaction with the nervous system in the case of MS (omega-6 is the building block for the pro-inflammatory cytokines. Both synthetic routes use a common catalyst in the body and what can happen is that if you eat too much omega-6 (from, say, canola) that can block the conversion of omega-3 by saturating the synthetic pathway).

Yes simple carbs are a problem with diabetes / syndrome X. But the simple carb story is not the only important nutritional story that needs to be told.

The point I'm trying to make is that "fats ain't fats". They are biochemically active and have different nutritional functions in your body. They are not just inert energy carriers (although they serve that purpose also).