We currently think that nicotine has a protective effect on the brain. Research is spotty but promising. Google “nicotine protects brain” to get started!
Oh, I'm certainly under no impression that nicotine might not have any health benefits, but with various inconclusive research it's hard to assess the complete picture and net effect. I think it's partly this way because nicotine has been studied quite little disconnected from tobacco research. (It is my understanding that there is indeed no conclusive evidence of nicotine being greatly harmful to health, and I do use nicotine products myself.)