There are no plans to have giant wind farms in Gauteng or anywhere else inland - that would be ridiculous as there is a general lack of wind there thus making it a wasted investment. Almost all the wind plans are in the W Cape and E Cape - where it is generally windy. Thus if the wind is not blowing in those 2 provinces, no wind energy is generated.

But let's ignore the above and use your statement that we average "less than 2 windless nights PER YEAR". Hell, let's just make it one windless night per year! What do you propose we do on that night? Will we need a huge investment in gas turbines equal to our wind turbine capacity to be ready and waiting to work for just that one night every year? How much do you think that will cost? 

That is the crux of the problem: you need an additional backup system to operate on the windless night. You don't need this with baseload/dispatchable power as there is almost no chance that every nuclear/coal/gas power station will simultaneously fail.