Are you sure it's worth letting fire spread? Because you seem to need more faith to buy back cards to fix the tiles, plus the resources to build the cards acquired...
These fires are more obvious indeed.
you can place many building on the charred lands, and I might give bonus to farms (inspired by real life). Also I decreased the prices of the meadows and forests significantly.
But I'll think about it. I think the extra faith is mainly useful in the early game.
BTW, in the future, faith will not be used to buy build cards from the shop, but for something else :)