"Shutdown after Defrag" is perfect for many occasions such as going to bed, work, or leaving your computer for a long break.
There should also be "Standby...", "Log Off...", "Hibernate...", Switch User...", and "Restart after Defrag" for the rest of those who come back frequently, need to change users when sharing a computer, or restart for various reasons.
Adding these to the Settings as a list of checks is fine, or placing them into another drop down group would work as well.
Keep up the good work! I love Defraggler!