1 marijuana needs light to grow and does not require a dark period
2 you gain 42hr of light a week with the 24/7 over 18/6 almost 2 full days of growth per week.
3 Because the plant is growing faster you shorten the time to sexual maturity
Shorter bushier plants have more bud sites and therefore, has more bud.
Unless you have terrific heat problems that you cannot control in the middle of the summer during an extremely hot period, I see no upside to running lights 18-6.