Wouldn't it be a bit onerous if you renamed/encrypted your wallet.dat file on a USB and then later on want to perform a quick transaction with someone?
The steps of decryption, renaming and placing the file back in the proper file under the Bitcoin client doesn't seem conducive for regular usage.
Would a paper method be as time consuming? Sorry but I'm addicted to efficiency and also plan for years down the road when maybe I might forget how to manage all the steps.