I definitely agree that it's much easier to clean a water dish than an entire terrarium! I also think that this only reaffirms just how clean reptiles are. Even a lot of our snakes tend to defecate in their water bowls. One only needs to observe our lizards and/or snakes for a while to realize how clean these animals are. Their behavior dictates it -- lift tail, defecate, move away while tail is lifted, and move on. They really are very clean. It's up to us to keep up with cleaning their environment. 
Whenever we want to take one of our iguanas out for some "social time", all we need to do is put them in a tub of water and wait... inevitably, they will defecate, then we are assured that they will not do it on our sofa or our rug... Not sure if this applies with *all* other reptiles, but we have noticed this behavior with some of our other reptiles as well... It has proven to be very convenient. LOL
Julie, Justin and the croc skinks, who have been laying eggs lately. 