I think that the technique of using the axe as a "hammer" may have come from way back . Years ago "carbon steel" was rare and expensive , so blacksmiths would forge weld a strip onto the edge of a piece of iron to make an axe head (the carbon steel being the "hard" blade). A chisel was made in a similar manner , leaving the soft end for hitting (hence the mushroom effect from repeated use) . This means that neither items would shatter or chip .
Today it is far easier and cheaper to make the whole axe head from carbon steel , so the back of the axe head is likely to be hard .
I personally dont like the idea of the blade going past my ear (i like them where they are) , or worse imbeeding it in my forhead , so i never reverse an axe or use double edged axes , bilhooks or machetes .
But if you HAVE to bang steel on steel , use a piece of wood in between . It might be less efficent , but its a lot safer .
Pumbaa