To have a 600V system in the US, or with US manufactured equipment becomes difficult because 600V is the limit for many ratings of equipment. If the system were to go 10% above nominal voltage (many things could cause this) you could easily be above the 600V rating. Not only on equipment but cable insulation as well. The break point according to ANSI C84.1 is 600V for Low Voltage, anything above that you are looking at medium voltage rating. For this set-up you really wouldn't want a nominal system voltage much greater than 540V.
Outside the US, where the predominant standards to follow are based on IEC, the break-point between LV and MV is 1000V, and the equipment as well as cable is rated up to 1000V. Therefore the likelihood that you will see a system with a voltage of 600V (or higher) becomes much greater, with the actual voltage based on in country standard practices. Typically, from what I have experienced, the voltages above the 400's are used for motor loads where the motor size puts a strain on the LV bus, but would be costly to purchase as an MV motor.