Why are silicones considered bad for hair?
Silicones can be drying, I think. I read somewhere that they can dry out your hair if you use too much and don't wash them out. They're not water-soluble, meaning you have to use harsh sulfates to get them out of your hair.
Check out this video, she does a good job of explaining. Skip to 2:16 for the info.https://www.youtube.com/watch?v=zwNmK6hDFqQ&list=UUD0XqR_pfjolXudnjkeHgIw
They are bad because they coat and weigh down your hair, causing long term build-up and the only way to get them out is to use harsh chemicals like sodium laurel sulfate. Also, they don't really moisturize your hair, they just make it appear moisturized and healthy.