It's more efficient if you plug it into an existing plant which is burning fossil fuels anyway (maybe even an existing natural gas plant, which if the loop was closed would basically make it a really stable chemical battery, which isn't really defeating the purpose at all!).

But, if you read the wikipedia article, you can see there are prototype plants using ambient air capture. It's probably a bit less efficient, but since it's actually reducing carbon levels, it's even better than just a battery.