If you have physical access to a machine, it's pretty straightforward to recover the root password thereby skipping the whole point of having an encrypted drive.
Am I missing something here?
EDIT: It seems that I was mistaken in a fundamental concept of disk encryption. The password for disk / directory encryption is separate from that of the system account passwords.
In the case of Full disk encryption, access to any system password needs to first go through the encryption password.
In the case where only the home directory is encrypted, the user login password is used along with the encryption password to create phrase which is used to encrypt and decrypt the home directory. A forced change of the user password (from a root account for e.g) will not update this encryption phrase and thus the data will remain secure.