You are correct when WiFi is enabled data shutoff until WiFi is disabled then data connection search and connects, so IDK what the OP was talking about and where he heard WiFi was good for the battery as I never heard of it.
Matter of fact since radio signal under wifi is used in diff portion as not allocated in kernel it uses resource not directly related to the system itself. With that being said the connection if active will use resource as in power.
Some uses app's like JuiceDefender which will turn off data/wifi if screen is off or based on setup will push connection via wifi/data in given interval. But you don't need app's for that as you can simply have it disabled when screen is off and resync when you turn on the screen. I know this very well as I use 90% time wifi for my data.
^ SS I just took now and posting it off phone via WiFi hell this is just stock battery LOL my Mugen Power didn't even come yet the stock size and that post to be 20% more better than stock. If you think thats efficient then wait till you see what I pull.