Because buffered windows are required for transparency or non-rectangular shapes, almost all Cocoa windows are buffered. Nonretained windows are appropriate for transitory images or simple connection lines, such as are used in Interface Builder for target-action connections.
http://developer.apple.com/documentation/Cocoa/Conceptual/CocoaFundamentals/CoreAppArchitecture/chapter_7_section_4.html
Anything is possible ;)