I haven't used Sparrow, since I don't have an iOS device, but any particular reason why Google thought they needed to buy it? What could Sparrow bring to the Gmail mobile app that Google couldn't do themselves (or well enough)? Or maybe they just wanted it so the competition doesn't get it first, just like Facebook wanted Instagram?
Sparrow was the only native app that managed to offer a better Gmail experience than the Gmail web app. Google's desktop and phone gmail apps have been generally, not great. Sparrow's was.