My impression (from talking to engineers at my local electric utility here in the southwest US) is exactly that: renewables can be unpredictable, and not always available when demand is high. NG generators, unlike coal or nuclear, are relatively easy to spin up on demand. (Perhaps someone more knowledgeable out there could fill in details.)

I do think there are many experiments with different storage technologies to smooth out supply, so this particular advantage of NG could change in the next few years.