What they say is that wind needs to have complete back up for those occasions when the wind is not blowing. The argument is intuitively appealing but factually incorrect.
What experience actually shows. Very high wind uptake in multiple jurisdictions around the world, together with detailed electro-techncial studies compiled by major companies such as GE, demonstrate that there is no technical nor economic reason why wind energy should not supply at least 20 percent of North America's electricity. Combine that with solar and the amount rises beyond 30 percent.
Dealing with variable wind (& solar) is about changing grid management procedures and practices. High penetration of variable renewables really is possible using little more than a variety of sophisticated, low cost, grid management techniques.
This may include improved wind energy forecasting, 21st century grid management techniques and better use of existing hydro resources. Strengthened electrical interconnections with neighboring regions (Manitoba, Alberta, North Dakota and Montana) should also be considered.
An example - better use of our existing hydro-electric capacity: Saskatchewan's existing hydro resources are constrained in their current usage because of limited water availability. With increased wind energy usage - we can store water when the wind is blowing and then use that same water to generate electricity when there is no wind.
Another example - wind & solar work well together. The windiest time of year is Fall, Winter and Spring. The sunniest time of year is (surprise!) Summer. Experience from jurisdictions that already have significant wind and solar (Germany) shows that together these two resources provide a relatively stable supply of electricity. Residual volatility can be smoothed using our abundant hydro-electric resource.
Jurisdictions already far beyond 20 percent wind. Proof that it is possible to have more than 20 percent of electricity from wind energy lies in the fact that two US states (Iowa and South Dakota) already generate close to 30 percent of their electricity using wind turbines.
The US is planning for 35 percent electricity from wind. So confident is the US Government in the potential of wind that in May 2014 the Department of Energy released its 'Wind Vision Initiative' in accordance with which the US will source 10 percent of its electricity from wind by 2020, 20 percent by 2030 and 35 percent by 2050.
If you'd like more information check this blog post.