Yeah only about 95% of the population. If you're one of the few extremely disciplined people that would spend the exact same amount whether you paid with plastic or cash then no it won't matter. The reality is that very few people are so disciplined that they only buy what they need.What you're saying may apply to a certain sample group, but you're ignoring the fact that not everyone is bad with money. It may have been observed that a group of people as a whole spent less money when paying cash, but that's not a very useful "fact". Correlation vs causation, right?
People who are great with money and pay their balance every month will still spend more over the course of a year than if they used cash. The average is 18% more a year so someone who is extremely disciplined might only spend 5% more a year so the person who isn't very disciplined will be way over 18% to get the average back down to 18%.