Plant's grown indoors are VERY sensitive to light, compared to their outdoor neighbors. A indoor plant can become stressed with even a little light. This can delay growth, cause hermaphrodites and reduce yield.
I understand your argument about it being nature. But an indoor plant isn't nature. The reason for it is that you can't reproduce the suns light. You could have a million watts worth of lights, but it still doesn't compare to the sun. The sun produces 1000 watts per square meter. It emits everything from radio waves, visible light, to gamma rays. So it emits ALL possible visible light frequencies. Even the strongest HID's emit light at certain frequencies. Some are more usable than others at different times in growth. That is why people use MH for veg and HPS for flower. You must also consider that even a 1000 watt HID doesn't produce anywhere near 1000 watts per square meter. So indoor plants are already starved for light, they will use anything they can get. Combine that with the heat it would produce to make 1000 watts per square meter, the sun wins by a long shot.
It is also hard to create the conditions found in nature indoors. No plant goes from 24 or 18/6 hours of light directly to 12/12. It takes months of losing 1 minute per day for the transition into flowering to occur. Co2, wind, rain, bugs...even the most high tech grow rooms don't come close to nature.
All this makes indoor plants extremely sensitive to stress. If a small amount of light gets through, you will almost certainly stress your plant. If it will turn hermie, stunt growth or delay flowering all has to do with genetics.
Short answer. Make your grow area light proof. Your plants will thank you. It doesn't take much. Remember, light only bends twice. Black plastic and tape is available at any wal mart. I bought 100 square feet and tape for less than 10 bucks.
