Standing at a concert one person soon realises they can see better by standing on their tip toes. In response the person behind stands on tip toes to be swiftly followed by everyone else, just to revert to the view they had to start with. Benefit over! For everyone!

Now think vouchers.....

The first discounter gains an initial advantage but soon their activity encourages competitors to do the same (they feel they have to) - the net effect is soon no competitive volume advantage for anyone just ...