They already use an archaic form of money. However, the idea of a currency that represents its face value and is forced to have a limited supply sounds more reasonable when you think about the non-stopping there. Accessing the internet or using bitcoin through a third party mobile service provider sounds unreliable and probably is a lot more inefficient for such a state. As of receiving payments and donations in bitcoin, sure it's easy to receive, but good luck laundering it while being sanctioned by nearly the entire planet. Self-sufficiency is important, imo bitcoin wouldn't provide that to ISIS.