That's a good point. I've never compared the battery usage. But you don't have to make everything 3D (I think). I think you can use WebGL in a 2D context and the battery impact is probably less significant.
Mostly, I just want to avoid learning each mobile platform just to make one app. That's been turning me off from mobile development for years. I'd rather use something like Cordova with webgl and hit all mobile platforms in one shot.