Some renewable energy sources (not all of them) are indeed producing electricity in a variable, discontinuous, and hardly programmable way, as they depend on weather conditions and night/day cycles. This is the case for wind and solar energy. However, our weather forecasting knowledge is now sufficient to anticipate supply variability.
The issue of variability is not critical at the level of one production plant, but needs to be considered at the level of the whole electricity grid. On a territory like France, the wide geographical distribution of renewable sites smoothens the effects of local variability. Wind turbines do not brutally stop by the hundreds simultaneously.
In fact, big thermal power plants can be seen as more intermittent in a way, wen taking into account unexpected interruptions. French nuclear reactors have experienced 260 of such production dropping events in 2014, i.e. five per week (source : RTE, the French electricity grid authority).
When properly combined, renewable energies do contribute to reducing the need for fossil and nuclear plants, as acknowledged by RTE : ‘Wind (and more generally renewable) energy production (…) limits the need for thermal plants in Europe.’