Exactly, it might work for one person, but not for the other person. I have buddies that are still convinced that they will always end up winning with Martingale. Nomatter what I say or even give my own personal examples, they still think that they will end up in profit...
Martinfail is never recommended to players, but those who are new in gambling and think they can outsmart the casino by methods written on the internet which have constraints that cannot be fulfilled in reality. They forget that the casino is running for years and have been making profits for many years too from the players who lose more than win.
One method to teach them is allow them to use the dummy paper gambling method to play and show that in the long run the balance will turn red.