Okay, I can accept that you have your opinion on the subject that differs from mine. I just decided, mainly after talking to my vet for a couple minutes, that it isn't worth the risk. Yes, most of the time there are other factors involved, but that doesn't affect the way a chronic impaction builds up. It is impossible for all of the sand to pass each time they ingest it, so a little will stay in the intestines. If ingesting sand only happens every once in a great while, this usually won't cause a problem, and sand will slowly work its way out of the system. If however, the animal is getting a little bit of sand in the regularly (from licking it or eating off of it), it will continue to build up very slowly until it either blocks off the intestines completely, or just makes it impossible for the animal to digest food. Even if I had a leopard gecko that was in perfect health, I still wouldn't trust myself to put it on sand just because it's a risk that I find unnecessary. It's just a personal preference. I don't want to lose a pet because of such a careless mistake. I would honestly think twice about putting a bearded dragon on sand ever again too. It's not as much that I'm worried about impaction, but they just aren't designed for it. If you've ever seen a bearded dragon walk on sand, you can tell that it's a little troublesome.
I think that was another of your questions too right? How are they not built for walking on sand? I'm not sure I can answer that for leopard geckos, because I don't have one yet, and i don't think I'll ever put one on sand, but I can give you my input I've accumulated from my bearded dragon. I noticed that when I used to have sand in his cage, he would stay up on his branches much more, and when he did come done, he seemed much less than graceful on the sand. His feet sunk in and sand was getting thrown everywhere (including his food dish). I switched to using either carpet or rubber shelf liner (and I might try slate tiles or even packed dirt in the future), and he spends more time running around on the ground and seems to enjoy running around the cage more. Plus, his food is always clean now.
A leopard gecko is obviously a smaller creature, so it's hard for me to picture them running on sand anyways (don't know if they'd sink in or not), but my preference is simply to not keep my reptiles on sand. It's messy, dusty, and can cause problems. That's just my opinion though.