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We consider multiuser MIMO systems with some of the users having target-rate requirements while the other users are working on best-effort data transmission. Both multiple-access channels (MAC) and broadcast channels (BC) are considered. We show that the best-effort users can achieve higher rates when the data streams of any user are not restricted to be decoded consecutively and when time-sharing between different decoding orders is allowed. For MIMO-MAC, we show that the problem can be formulated as a convex optimization problem, and hence efficient algorithms can be used to obtain the globally optimal solution. We also show that in some cases it is possible to reduce the transmission power of some users without affecting the optimal solution, and we show that the power minimization problem is convex. We also exploit the MAC-BC duality in order to solve the same problem for the MIMO-BC channel.