The conventional wisdom is that hydraulic or fossil fired plants can be operated at less than full power to accommodate the wind generation.
From my reading, stable operation for a fired thermal generator is normally from about 50% of the unit rating up to full power. For reduced power operation some units add oil to supplement the coal feed and maintain stable "firing" conditions. Thermal units range in size, but are typically a few hundred MW.
EON Netz report that wind generation in excess of 20% of the grid demand can lead to instability conditions due to variability. Another real condition arises on those occasions when the total output of the installed wind generation is very small. EON Netz, with 7000 MW of wind generation, rely on only 8% in their supply in planning. This means they can count on only 560 MW from the wind turbines, and must have sufficient "spinning reserve" to accept rapid loss of wind generation capacity. Hence, there must be a number of thermal units running at 50% of capacity (or more) to be available to respond.
Alas, heat engines running at 50% are usually less efficient than those running closer to their unit rating. (This is one aspect where diesel engines perform well.)
From the point of view of meeting the power demand, including the installed nameplate capacity of wind generators is misleading. The Ontario supply plan had better include sufficient energy import capacity to provide a real margin on the load demand rather than a mythical one.
Another aspect of electrical generation that isn't often mentioned is "VAR generation". The whole issue of managing reactive power loading on the distribution system is handled better in Europe than in North America. European standards require the owner of the load to control their power factor (phase angle between voltage and current) and harmonic characteristics. In North America these characteristics of loads are less well regulated, leaving the utilities and the customers in a confused state about whose equipment did what, and who pays to condition the power.
Anyway, most wind generators aren't capable of generating "reactive power". (It would be a cheap shot to say that they are good at generating imaginary power, so I won't.) Some are being equipped with solid state equipment to improve on this aspect.
The wind is free. Wind generated electricity isn't. Not by a long shot.