Like Tomte and WingItRanch, I have seen some of mine eating ash and possibly charcoal (there are often little pieces scattered about in the ash after a bonfire) when dustbathing in it, and they were none the worse for wear afterwards.
If you offer it separately then (1) they can choose whether or not to consume it, rather than mix it in with whatever you feed them and thereby sort of force-feed them it, and (2) you can observe any differences in the health of those that choose to eat it versus those that decline it.
