Endoran,
I use Juice Plotter along with Ultimate Juice.
In the last few nights, I had UJ disable all mobile data, but keep its connection to Wifi, from midnight to 6am.
Unless I use Wifi to surf during that time, there is absolutely zero battery drain, if Juice Plotter is to be believed. The plotline is a straigth horizontal line from midnight to 6am, and the battery percentage is the same at 6am as it was at midnight.
I know if I kept a wifi connection all night with my former WinMo device, my phone would have been dead by morning. But not so with my Captivate. I think I've read that's because the new wifi technology are better engineered now.
So, for me, anyway, maintaining the wifi connection is not draining battery "at rest".
It is possible that if I kept wifi on while out and about, and it continuously seeks a usable wifi connection without finding one, that would drain battery...or if my wifi signal was weak.
Currently, I either automatically shut down wifi with Tasker or do it manually when I realize I left it on by accident. My wifi never sleep policy is just to maintain a wifi connection when the screen is off, so that it doesn't accidentally revert to 3G (which it did before I changed it to never sleep).
My 2 cents.