Penetration:
An LED is providing 1000 umol to the top of a plant from 6", double it to 12" and you have 500 umol. Putting this over a 24" plant ignores the lower buds.
Take the same plant getting the same 1000 umol on the top, except from 15" away. Double it to 30" and you have 500 umol. Putting this over a 24" plant will totally kick ass all the way to the bottom.
That is how penetration works. But since it costs so much for power to get 1000 umol from 15" most get a whole lot less power, put it really close and get a good bud on top and popcorn below.
This is a 50" plant at 50 days, note the size of the lower bud, more than four feet from the light. The sidelights did not make the bud big, UVB stunts growth. The LED's provide 800 umol at 18", the max the plants can handle in the system I use. That is still 400 umol at three feet, more than the max of some budrooms I have visited.
It depends on the goal of the gardener, the extra 15% of yield per square foot of grow area make it worth it to me. Growers that do not have to heat up the growroom during -45 degree ambient temperatures may have more growing space available and this will matter less.