As DJ9 mentioned, a bios password is useless. If you have physical access to the box you can circumvent it.
A root password is equally useless. If you have physical access you can override it.
Shares passwords are the same thing. Physical access negates the need to connect via a sharing protocol.
Drive encryption with passphrase needed to mount defeats the physical access thing. Even if it's running at the time, properly secured, any attacker is going to have to reboot the box to gain root access, etc. And since a password is required to mount after every reboot, this verifies the box hasn't compromised or stolen.
The only advantage I see of having encrypted drives without a password (auto-mount on boot) is that you can RMA drives without fear of any of the data being readable.
Thinking about it, if I was running drive encryption with passphrase, I'd make sure to disable the quick launch menu thing that freenas helpfully throws up. As that is an easy way to get root access to a running machine. Assuming you have local console access to begin with. (I don't bother on my nas's. Nothing on them is important enough to encrypt. Even without the quick menu, a simple reboot would be all that's required to gain root access.)
If the box is physically secured (ie datacenter with 24/7 security guards, etc), then I can see not wanting a passphrase and wanting the pool to automount. Then you don't have to worry about wiping drives before rma'ing them.