It all depends on how reliably wind farms can commit to a specified production for up to the start-up time of the back-up fossil fuel plants. Otherwise, the fossil fuel plants must remain in hot stand-by with a very high fuel consumption while doing nothing.

The "high fuel consumption" is all relative - it's high per kWh produced, but no so much in overall terms, so it's an acceptable trade off.

And you might find this text interesting.

In the long run, we're all dead. John Maynard Keynes