They only work when the wind is blowing, and when the said wind dies down, the only thing that can take it's place is thermal, be it coal, oil or gas fired. Trouble is, you can't just switch on and of a thermal generation station. It takes days to get them up to temperature and pressure.
You can switch them on, you know. It all depends on whether they are being kept as spinning reserve (turning, synched, not generating), warm reserve (boilers just about simmering) or cold reserve (what it sounds like). Spinning reserve can come on line in a few minutes, warm reserve in an hour or two or so, cold reserve in a day. Gas turbine plants get up to full output from cold in twenty minutes or so. Weather forecasts aren't great, but they are good enough for that sort of notice, so it's certainly not necessary to keep thermal plants running all the time in case they are needed.
Sure, few renewables give constant output - tidal stops twice a day after all - but we're just going to have to cope with that. Every kWh generated from renewables is a kWh not generated from fossil fuels (nukes run at full tilt all the time for physics reasons) and that's a Good Thing.