If the feds seize your money, well then you are either into some heavy criminal activity or you have owed them a large amount of back taxes for a very long time.
Because law enforcement never makes any mistakes, and malicious prosecution never happens?
When your bank accounts are seized then most people no longer have any way to pay for a good defense attorney, if you have enough bitcoins stashed somewhere safe you could still have the resources to defend yourself.
Also, where does the average bitcoin user keep his wallet/private key? If it is located on a computer they have on or around them, isn't that able to be seized as well if the feds come for your private property as well as your bank account, which is often the case and certainly would be if they knew you to be a bitcoin user engaged in illegal activity?
Best practices would be to have multiple backups in multiple locations.
You could also just go with a brainwallet.
https://en.bitcoin.it/wiki/Brainwallet
Brainwallet: The Ultimate in Mobile Money - Forbes