Modern on-chip networks (NoCs) rely on virtual channel (VC) flow control to allow effective utilization of link bandwidth at the cost of more power and longer per-hop latency. Despite many existing optimization techniques for NoCs under VC flow control, we take a further step on questioning its necessity. Our finding is, when the network is not busy, circuit-switching (CS) may already satisfy the performance requirements with much smaller power consumption and shorter per-hop latency. In this paper, we propose to opportunistically enable CS in NoCs under VC flow control. This allows us to effectively reduce the power consumption of NoCs through having less buffering and longer sleep intervals for power gating while retaining CS-like per-hop latency. Our evaluations reveal that this proposal leads to a reduction of network power by up to 70% while cutting the system energy footprint by up to 35%.