It seems from the recent Science paper by Altwegg et al (2014) that the commonly accepted source of Earth's water being the comets might not be (completely) true. Their study suggest that the water in the comet they studied is different from Earth's (higher Deuterium content). The alternative source of water that Altwegg suggests is from asteroids. My perception is that there is not that much water in asteroids or at least not sufficient to create Earth's oceans. How much water is in asteroids now? How much water was there in asteroids reaching Earth when the oceans were formed?
This is far too soon to draw sweeping conclusions. The Philae probe has only measured deuterium for one comet. But comets are diverse, of diverse origins. We need to take a lot more samples, from many different comets, before we can conclude firmly that water has, or has not, come from comets.
The only conclusion we could draw so far from this study is that earthly water has not come exclusively from comets similar to 67P/C-G.
What you're seeing now is the usual distortion that occurs in the media after some hugely popular experiment is completed - its conclusions are blown out of proportion. But that's not how science works.
Let's write this one down, and wait for further science to be put forth. We don't know enough yet about these objects.