The issues with Hot rocks stem from the very manner in which they are designed. yes, reptiles do use belly heat and it plays a very important role in digestion. the issue stems from their basking behavior. in the wild, they head out to exposed outcroppings where the source of heat radiates from the sun downwards. this is also the reason why under tank heaters don't work well for diurnal lizards. that's how they are programmed to measure their exposure-from above. hot rocks not only are largely unpredictable, they don't provide heat that heats up the entire terrarium. the greatest problem is lack of choices. in a properly heated cage, a dragon has access to a range of temperatures to bask at, just by varying its position away or closer to the radiant heat source (in this case, a light bulb). this lets the animal choose. in an enclosure where the only source of heat is a hot rock (as most of the time it is) they are forced to choose between cold and hot. cold may be too cold, and hot may be too hot or not hot enough. the chance of malfunction is the greatest danger however, and as the other poster has shown, the results are often very gruesome. at a more basic level, a light bulb and a normal flat rock are a much cheaper than purchasing a 30-40 dollar hot rock. it just doesn't pay.