Consistent output is certainly useful when you break down demand into a constant demand + variable demand. For instance, if demand is typically 200-350 kWh, you could build a nuclear plant to generate 200 kWh and constantly run while you meet the varying 0-150 kWh demand with wind and solar.

I will agree though that we need to run numbers on this to determine the best approach. I don't have a feel for what wins out if we make larger solar and wind farms -- increased cost for the additional capacity, or increased efficiency from economies of scale.