When you have a laptop which requires ">1 day" of defragging, it is not advised to leave the power plugged in for that long.
If you defrag on battery, you have to be constantly available to "revive" it.
If you change the power settings to remove sleep on idle and are away when critical battery level is reached, hibernation seems to be safe enough for defraggler. The problem is remembering to return the power settings (and what they were) afterwards.
In an earlier thread, a sysadmin asked about desktops set to sleep on idle sheduled to defrag at night.
Is it possible for a non-default option of defraggler to temporarily change the power settings in Windows 8.1?