Whether fracking for gas really is a bridge to reduced greenhouse emissions depends on context - does gas replace coal, or not?
That may seem too obvious to be worth discussing, but it's helpful to me as a matter of geography and chronology, especially chronology. The reason for time being important depends on whether you accept that the long-term trend of significant decline in solar power costs, faster than efficiency for coal, will continue in the future and reach grid parity. Similar evidence for wind, if not quite as dramatic. I'm mostly buying these arguments.
On the geographic scale of the middle and eastern US, fracking has clearly replaced coal, and seems beneficial from a climate perspective (ignoring the other environmental issues). In the western half of the US and much of western Europe, coal is much less important a power source, and gas from fracking seems more competitive with low emission energy. When you add the chronological aspect that fracking will take 5-15 years to really develop, the same time period when renewables are approaching grid parity, then the argument for its development seems a lot shakier.
Exporting gas from the US will also take a decade or more, so again from a climate perspective, that only makes sense if the exports replace reliance on coal. Maybe in China, India, and other developing markets, the climate would be better off if they had more gas. Exports to Europe would be bad, I think. Not sure where the gas is really anticipated to go.
Fracking is an emerging issue here in California, where we have very little coal use to displace. And even more locally in Santa Clara County, we've got lots of shale, where we've stored in underground aquifers a year's worth of drinking water for 1.8 million people.