I know they are unnecessary, but they look so cool!
Different wallpapers drain the battery differently. It really depends on how many different factors the wallpaper is paying attention to. Is it just being pretty or is it altering itself based on every movement of your phone?
The best way to find out is to simply test it yourself. Spend one charge of your phone running a live wallpaper. When you get near the end of the battery charge, open up your phone's settings. Go to "About Phone" -> "Battery" -> "Battery Use". This screen will show you all services that have used up more than 2% of your battery's charge.
Your first reaction will probably be that it is taking up a lot when you see "Display" draining the most but that isn't taking into account wallpaper. The live wallpaper, if it has used more than 2% (some don't), will show up as it's own service.
They will pull more than a normal wallpaper (that's a given) but some won't effect it the battery charge nearly as much as say your widgets or the brightness of the display. Turn down the brightness and you could probably offset the difference in battery charge of running a live wallpaper.