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When fast and accurate channel state information (CSI) at the transmitter is available, opportunistic power control (OPC) is an attractive alternative to signal-to-interference-and-noise ratio (SINR) target following approaches, because it maximizes throughput by taking advantage of fast channel variations. On the other hand, in multiple-input multiple-output (MIMO) systems, open loop schemes such as Alamouti space time coding and spatial multiplexing (SM) exploit channel diversity without accurate CSI. In an attempt to answer the question whether OPC is worth the pain of obtaining fast CSI, in this paper we evaluate the gains of OPC for the downlink of a system employing MIMO with Alamouti and SM. We formulate the OPC problem as a throughput maximization task subject to a power budget and fairness constraints. We solve this task by the Augmented Lagrangian Penalty Function (ALPF) and find that without fairness constraints, OPC in concert with SM provides superior throughput. With tight fairness constraint, Alamouti along with equal power allocation becomes a viable alternative to the SM OPC scheme.