I dont think it will consume more battery. Previously, when your phone sleeps, the wifi disconnects and your connection goes back to data, so you are still using data, which does consume battery. Honestly, you dont have to be worried either way, battery will be consumed in idle mode, but very less, because you have an option to close applications in sleep mode.
Regarding the update, it is definitely welcome. It is always better for the use to have control. I have a few friends who dont use data at all. They have a 200 mb data connection, and they switch on the flight mode and use google voice and wifi. So, it was increasingly fustrating to see their wifi being disconnected when the phone sleeps.
As for myself, I use my office wifi, but can use it only after I authenticate from the browser. So every time my phone sleeps, I had to authenticate from the browser everytime.
And as I said, I would prefer that the phone uses my wifi during sleep mode than my data connection.