Think about this. If you ate just one multi-vitamin every day that had just the right vitamins and minerals and you got all of the nutritional need, would you survive? No. You need substance, you need to eat so much every day. However, eating a multivitamin is very good for you health if you still eat enough substance food.
The same thing applies to LED's. They are like the multivitamin. They give the plants just the right amount of light, but just not enough intensity. They are great for side lighting or additional lighting in a box grow, but only in addition to HID's or CFL's.