With the realease of new runes, everything is adaptative. It depends on the champion you're on, on the items you bought. So I thought why not make Infernal drake's bonus adaptative as well ? Either 8% AP, or 8% AD ? Or maybe tune it up, since mages wouldn't be getting the free AD and vice versa.
I thought it could be an interesting tweak, and wouldn't have too many consequences, other than consistancy with runes.