MIMO radar employs multiple antennas to simultaneously transmit diverse waveforms, as well as multiple antennas to receive the radar returns. This paper studies the role of waveform diversity in MIMO radar as separate and distinct from the role of the multiple transmit antennas. This is done by comparing a MIMO radar system to a scanning phased-array radar which uses the same transmit and receive arrays but only a single waveform. The performance characteristics of the two systems are compared in detail for single pulse operation as well as multi-pulse operation with coherent integration. Both element-space and beam-space systems are considered.