You do realize that wifi uses less battery than 3G, right? It's like a game of pitch and catch. When your friend is only a few feet from you, it doesn't take much effort to pitch the ball back and forth. But when both of you are 50 feet or so apart, it takes more effort to pitch the ball that distance. Same goes with wifi / 3G (4G, too). Your wifi router is in your house. The cell phone tower isn't. That is why wifi takes less power. It doesn't have to transmit at a high enough power to reach a cell phone tower that could be over 10 miles away. Your 3G radio does. 4G also eats your battery because, not only does it have to transmit to an antenna that could be miles away, but it has to support higher bandwidth.
If I were only using wi-fi, sure. But seeing as I intend to be able to make or receive calls while using wi-fi for data, I'd be using both, which I presume uses more energy than either by itself.

