Please present both sides of the issue.  If you use the same Electrum wallet you only need to back it up once.  Also, a thief only needs to steal your deterministic wallet seed once.  I advise Electrum users to occasionally (at a time you determine, depending on your amount of coins and tolerance of risk) to make a new Electrum wallet, back up the new deterministic seed, and send all coins to it.   If you import keys from a non-deterministic client, you need to back up those keys, or the whole wallet that contains them.  If you prefer going to bitcoin-qt as the main client, you should be able to import the individual electrum keys into it too.