This way there will be only one password to decrypt the system and you will
have to tell it to anyone who needs to use the notebook.
With an USB token login I can make for example 2 public/private keys and
generate 2 tokens. One to my login with full system access and another to
guest access with limited privileges.
That is the way that software works in Windows. I could not find any path to
make Ubuntu work like this in Google neither in forums.
The closer I found is an implementation of a PCMCIA token to sign e-mails (
http://www.linuxjournal.com/article/4744).