If you turn off windows themes, you probably won't have aero running, which will mean the windowblinds skins won't look quite right. Also, your taskbar previews won't work.
Very good point and from experience I agree. I tried turning off Windows Themes and not only did I not see any resources, per say, decrease much but many of the unskinnable areas looked like the old Windows theme. With Themes enabled they had the Aero, or whatever Windows theme you have enabled, properties.
Just an FYI.
Joe