Any benefits of nicotine have yet to be proven conclusively in human studies. This article takes a bit for grantedhttp://slatestarcodex.com/2016/01/11/schizophrenia-no-smoking-gun/ …
@PolycrystalhD Re Parkinson's, I'm working off papers like http://journal.frontiersin.org/article/10.3389/fnagi.2014.00340/full … and http://www.sciencedirect.com/science/article/pii/S0166223604001924 … which seem pretty convincing