As a key component in dynamic wavelength-routing optical networks (WRONs), the contention performance at add/drop (A/D) ports of ROADMs has attracted a lot of attention in recent years. For the first time, this paper derives the close-form solutions of the blocking probability (BP) of A/D requests to characterize the fundamental tradeoff between A/D flexibility and system complexity in the ROADM. We show that the ROADM with fiber cross-connect (FXC) can decide whether a request can be satisfied based on global transceiver-usage information, and thus an exact expression of BP can be obtained. In comparison, the ROADM without FXC needs detail transceiver-usage information to make decision, and thus is hard to be analyzed. To circumvent the difficulty in analysis, we describe the evolution of the number of busy transceivers using a one-dimensional Markov chain, where the state transition rates are estimated from global transceiver-usage information. Based on this model, we obtain an approximate BP the accuracy of which is high enough for currently commercialized ROADMs and increases with the number of drop ports. Our analytical results provide an interesting insight into the tradeoff between A/D flexibility and system complexity, based on which we give some suggestions for system design.