So let me get this straight: you can actually play 0-cost cards without actually owning a hero from that sphere? Checked the manual for it, and the deckbuilding section doesn't say you can't. For some reason I must have assumed you needed a living hero to play those, even at 0-cost. Well, that does give us a lot more options for deckbuilding. All this time I've been playing it wrong (luckily so far I haven't played that much).

Uhm. Checked the rulesbook and found this (pag. 12, emphasys mine):

Cards with a cost of zero do not require a resource to be spent in order to pay their cost, but they do require at least one hero under that player’s control to have a resource icon that matches the card’s sphere .

Unfortunately you have been right all along you indeed must control a hero of that sphere even to play 0 cost cards.

Oops. Thanks guys. Must've read over that last week.

I guess I thought that they merely had the icon to be in line with the new scrolls that let you play a certain spell of that sphere.