Many people will do it until they see for themselves how it hurts the plant. Take a plant that has two identical shoots coming off each side of the main stalk with a fan leaf extending from the stalk at the base of each shoot. Cut ONE of the fan leaves off one side, but leave the fan leaf on the other side uncut. By the time the plant is ready to be harvested the shoot that still has the fan leaf intact below it will be at least twice as heavy and full as the one without the fan leaf below it. I learned this my first grow after I cut a bunch of fan leaves off when I put the plants into flowering, big mistake. I also found that the plants with fan leaves removed stretched a LOT more in flowering.