Ill start by saying that I used sand before with my male since the first day I got him until he was 5 years old. His story and more details are available in the history section(under construction right now though).
So why is sand bad?
Let us start by filtering the myth: Sand causes Impactions.
Impactions are caused when non digestible solids build up in the gut, not allowing other foods or liquids to pass, causing the stomach to burst, ultimately leading to death.
Dragons don't go around eating sand. Yes they do get some in there mouth while hunting but it would take a LARGE amount of sand to cause impactions. They would have to eat mouth fulls at a time over a period of time. So unless your beardy has a mental disability it's not likely to go around eating sand.
Sand to a certain degree helps with digestion in small quantities.
The facts: Humans walk on two legs. Walking on sand helps us build our stabilizer muscles.
Bearded Dragons walk on four legs, and are low to the ground. When they walk in sand their legs sink, making their stomach drag in the sand. This sinking takes allot of weight off their legs between steps, which in turn causes loss of muscle mass in the legs. The impacts of this muscle loss is very apparent when they are on hard surfaces, climbing rocks, stumps and logs become difficult. They get clumsy and lose their balance running across solid surfaces.
Sand creates big dust clouds in a terrarium, and those clouds linger for quite some time, not to mention making cleaning more difficult and annoying. Breathing becomes laboured because every time they move they create a dust cloud and breath dust particles.
Sand leads to respiratory infections, collapsed lung/s, and death in some cases.
This isn't a problem in the wild because of the open spaces, fresh air, and various terrains available, they don't spend all their days in sand so they aren't breathing fine dust all day like beardies in an enclosed terrarium, and NO, the Exo-Terra with the ventilation ports don't help with this.
So I strongly advice against the use of sand.