"Alzheimer's Disease. Some evidence also suggests that nicotine may have short-term protective actions against disease mechanisms that cause Alzheimer's. In fact nicotine is being tested in Alzheimer's patients. It should be strongly noted, however, that such effects, if protective, are short term and are most likely not applicable to smokers. In fact, the best current research suggesting that smoking makes little difference in the risk for Alzheimer's, and if it does, the risk for dementia is slightly higher in smokers. (Certainly, smoking can affect blood vessels in the brain as it does in the heart, increasing the risk for dementia from small or major strokes.)"
From http://www.reutershealth.com/wellconnected/doc41.html

And to read about how smoking leads to dementia and mental decline:
http://www.findarticles.com/p/articles/mi_m0FSL/is_6_79/ai_n6076828

So even if you don't care that you smell disgusting and have a high risk of cancer and force those around you to inhale your smoke and all the other problems with it, I don't think avoiding Alzheimer's is a valid excuse for smoking.