I have read the arguements on the use of LEDs and decided to do some research myself. I don't want to sound arrogant on the subject but This is my opinion from what I have learned. LEDs just don't do what is needed.
The key is light energy. In the chloroplasts within a leaf
light energy is used (like gasoline in a car to make it go) to power chemical reactions that are used to build the compounds that the plant needs. LEDs are low wattage lights, therefore they only produce low power light.
Yeah I know that LEDs are better for emitting higher percentage of PAR light but it is still
low energy light. If you break light down to its most basic unit you get the
photon. When a photon strikes a cell of chlorophyll it causes 1 electron to be released from its bond in the CO2 molecule which allows the CO2 to break its bond so that the carbon atom can be utilized in building carbohydrates. The now free electron is connected to a energy carrying compound called ATP which is the plant's "gasoline" that is used to fuel all other chemical reactions. (For those who fully know this info, yes this is oversimplified)
For every photon of light that strikes the plant on a green area, a
free electron is produced. When you shine a
low energy light on a plant that is a
high energy plant, it is like using a squirt bottle of water to put out a large fire. It is a low concentration of photons, which is why it takes multiples of LED lights to even get close to the concentration of photons produced by HID lights. Also, the photons produced by LEDs are only being pushed by very low voltage, so they don't travel far before they lose their energy.
If you use an HID, it is like using a fire hose to spray water on the same fire. You are literally showering the plant with a high concentration of photons that are driven by upwards of 5000volts so that they will go farther without losing their energy.
Will LEDs work? yes. But they don't work well enough in any setup to offset the lack of photons (light energy) without becoming prohibitively expensive. When they are able to produce LEDs that push 50watts of power from each one, then we will have something to work with.