I stand corrected… when the 4500c is being used in omni mode, it does not simultaneously send data to each sector. SM’s are getting allocated time slots one by one. You won’t get any beamforming gains in this configuration, but that’s beside the point. For MU-MIMO to work the radio needs to be used in 8x8 or 2x 4x4 mode.
As to why to use a 4500c vs. a 4500L connected to an omni… I think the main reason to use a 4500c is that you have much more granular control over the antennas used and physical characteristics (direction, tilt, mount location, F/B ratio, etc), along with being able to use much higher gain antennas… as opposed to a single omni antenna. Basically, better noise rejection, and concentrating energy direction.