Ich and other parasites don't need sand. They're breed on any surface. People quarantine without sand so it's easy to clean the surfaces, so that's probably where this theory has come from.

Bare bottom tanks are more common with SPS keepers, as they can crank up the flow, without sand blowing about.  Personally, I have a deep sand bed, which requires special care, but I love the critters and life, and all that comes with having sand.