It's absolutely a net gain when compared to running gas/coal only. But unfair to say that this produces 0 CO2, like it's often claimed. And this backup CO2 production is very far from negligible.

Similarly, the cost of running gas only should be computed as: cost of gas power plant + cost of gas. And the cost of the renewable option should be computed as : cost of renewable power plan + cost of backup gas power plant + cost of backup gas. You can't magically exclude the backup from the calculation. And suddenly the renewable option sounds way more expensive.