Beardies lick things to see what they are, and they eat bugs out of the sand that will then have sand on them, so they ingest the sand and are not able to digest it. The sand can collect in their stomachs causing them many digestions problems, impaction and sometimes VERY soon after, death. Check out this link
http://www.beardeddragon.org/articles/impaction/?page=3
And, it is very dirty. They poo in the sand (and so does their food) and it is not possible to clean it ALL out unless you take out all the sand everyday and replace it. This can lead to pinworms and a plethora of other unsanitary infections. Sand is just a bad idea all around.
I suggest slate tiles which can be purchased cheap at Lowe's, or one of the many rolled reptile carpets you can find at almost any petstore.
Good luck, and if you have any other questions, we are always here
Edit: Sand can also discolour their skin and cause eye infections.