Will it be easy to adapt those "special" dice to the usual polyhedron dice of RPGs (d6, d8, etc.). Say you wanna play the game and you're at your friend's house or something, you can get the sheets from the net or just writting down in a blank piece of paper, you friend also plays RPG so he has a few d6, d8, d10, etc... but no Skull/Axe/Sword/Drop/etc Warhammer dice. You can't play the game then?
If these dice are half blank half "picture" then you could use a common one for odds/even (or greater than half the die) or something else, but what if the ratio is not that easy. There should be an easy way to do it, heck, I think there should be in the rules how to adapt, you can't be expected to take these dices everywhere you go.