The persistent shopping cart option suggest that there is a separation in session of a logged in user by machine/browser.
This should mean that if the option is disabled, you will not see the content of the shopping cart that you entered at the computer at your work when you login at home (different computer and browser but same login).
However when testing is does not make any difference, a cart filled with a product under say firstname.lastname@example.org can been seen on any computer or device when login as email@example.com.
separation however would be nice when using single signon.
How or where a changes needed to make this work?