If I want to annoy people at FogCreek, I can just send them emails with random case ids. e.g. I can reopen cases like 123 remotely by simply sending a mail to them using (Case FB123) in the subject line.
I don't like this. It would be better to use the ticket, so that customers cannot forge the case id.
Some of our customers already managed to accidentialy break the case number (including spaces, using 1 because it was their first case) in emails and when filling out a form for error reports.