Are solar, wind and marine power too intermittent to be useful?

The Guardian

Professor Jenny Nelson [Physics] and Dr Robert Gross [Centre for Environmental Policy] writes - "Electricity generated from renewable sources such as wind, marine and solar helps to cut carbon emissions by reducing the need for fossil fuel power generation. Renewable energy can also diversify supplies, helping to make a power system more resilient to failures and less exposed to fluctuating fuel prices. However, renewables pose a challenge in the form of intermittency, as their output varies with the available sunlight, wind speeds and wave activity.

http://www.guardian.co.uk/environment/2012/may/29/intermittant-solar-wind-energy?newsfeed=true