You're not wrong that there's a negative correlation between smoking and obesity or Parkinson's, but that is not a reason to take up smoking. You're far more likely to die of cancer or heart disease as a smoker than you are not to die from Parkinson's because you smoked.
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"no possible health benefit with smoking"...as usual, it's more complicated.
See, for example, http://aje.oxfordjournals.org/content/155/8/732.full
for the negative correlation between smoking and Parkinson's disease, and
http://ajcn.nutrition.org/content/87/4/801.full
for some insights into the negative correlation between smoking and obesity.
Not to say that smoking is a good idea, it's not (actuarial data are clear on that), but your view of its consequences is inaccurate.
You're not wrong that there's a negative correlation between smoking and obesity or Parkinson's, but that is not a reason to take up smoking. You're far more likely to die of cancer or heart disease as a smoker than you are not to die from Parkinson's because you smoked.
Americans are pretty likely to die from heart disease anyways, smoker or non-.