When does it make sense
      to rebuild a RAID-5, or any RAID subsystem?

Our Answer:

Never attempt to rebuild a RAID unless ALL of the hard disk drive members are present, accounted for, confirmed to be fully functional individually, connected in the correct sequential order, and that the RAID controller also performs its POST and any other test routines flawlessly.  All subsystem components must faultlessly functional for a rebuild operation to succeed.  Rebuild operations forced to proceed under other conditions are doomed to failure, and frequently damage existing data, make data recovery effort more difficult, and sometimes preclude recovery altogether.