I suppose you could argue that some people smoke all their lives and live until their nineties, while others could get a smoking-related illness earlier in life.
I would hope that people could learn from their mistakes and try to live a healthier lifestyle, but I would also hope that the NHS would be there for those in need.
However, I would draw the line at a young person getting sick notes to get benefit because she wants to lie in bed and stuff her face with food. Then again, someone is aiding and abetting her - the GP to keep giving her sick notes and a friend or relative who fetches the food for her.