In my experience with my iPhone 5 running iOS 6.1, it holds the wifi signal for about 1 minute, then disconnects. In my experience, even if data is going in and out in the background, it will not cause the wifi to reconnect, and therefore even if I'm at home or at work, I am burning up my data allotment needlessly.
Maybe this is fixed in iOS 7, but there's no way I'm willing to upgrade in order to find out. I'm perfectly happy using Insomnia and it doesn't seem to be having a significantly negative effect on my battery life.