The greatest advantage NG has over coal is the ease and affordability of building Combined Cycle Generation which makes a power plant roughly 1/3rd more efficient than a coal plant. That easily outweighs any other variability involved in getting either form of energy out of the ground.
And really, the greatest environment abuse caused by Natural Gas in the US right now has to do with oil drilling in North Dakota where the desire to get oil out the ground has outpaced the ability to collect and process the associated NG produced and the feeble state government has failed to enforce regulations requiring it. The result is that probably about half of all flaring of NG in the US occurs in ND which is only producing about 1% of the NG used in the US.
You can actually see the flaring at night from space.
http://www.csmonitor.com/Environment/2012/0713/Thanks-to-North-Dakota-US-waste-of-natural-gas-grows-rapidly