I modified your survey slightly
1) is a back up necessary?
Yes / No
2) would a back up be useful?
Yes / No
if the answer to question 1 or 2 is yes,
3)
Must the back up be identical to the main camera yes /
no
4)
Must the bakc up offer different functionality yes /
no
Currently my primary is the K20D, with the K10D as a backup. More often than not the K10D is either with a different lens or with the second shooter (my wife). Beyond that, for situations where I am really paranoid about my backups, I take my trusty Sony F717. I've never had to use it, but you never know.