I figure the reasons for not having any substrate in your tank are... even if the bits of substrate are too big for your cham to eat by accident your feeder bugs would all hide in it. Loosing their gutloaded nutritional value the whole time they are awaiting their fate. Even possibly attacking and biting your cham once they become hungry. Not to mention substrate makes it WAY harder to keep your enclosure clean, thus harbouring bacteria. Cheers!