This feature will be helpful not only from usability point of view (as originally requested), but as a mean of overcoming paranoid spam filters.
More details in the forum:
http://forum.cerb4.com/showthread.php?t=2354
A couple ideas from the forums. He didn't specifically cite this JIRA issue but they all seem to be recommendations on ways to bypass the Support Center registration.
http://www.cerb4.com/forums/showpost.php?p=14765&postcount=1
1.) Append a URL in either Snipets or on Reply emails that allows the Recipient to click a link that will open a web page to view the Ticket progress, see their Ticket history, etc.
2.) View Ticket with MASK id only, or MASK id with email address? (Would make the above question work too)
3.) Automagically create client users when they email the desk, or submit a new ticket as a guest, based on email address (email address as user id, MASK as essentially a password).
New Feature
Open
Simpler way for customers to check on ticket statuses without creating a Support Center login