While it is well accepted that cooperative spectrum sensing will significantly improve the sensing performance, the necessity of cooperation is not sufficiently appreciated. In this paper, by analyzing the spectrum sensing problem from the system perspective, we show that without cooperation, the performance will suffer from a fundamental tradeoff between reliability and efficiency. However, if cooperation is incorporated in the spectrum sensing process and the threshold is selected appropriately, the efficiencyreliability tradeoff in the non-cooperative case can be largely overcome by exploiting the cooperative diversity. These results show that cooperation in spectrum sensing is not just a luxury but a necessity.