Robb, I share but educating is difficult
Joke aside, even if you don't want to apply quota at mailbox level (which I can understand although it may have some side effect, not only in term of disk space but also in term of backup and global performance), I'm convinced that limiting maximum mail size is a good practice.
Back to quotas: I don't know any (big) company that is offering "no limit" mailboxes for all employees. I don't know any neither that is applying quotas without exception for some key users (VIP) but Zentyal permits this very easily except if you have lot of users because exception is not managed by profile but per account.
@aspangilinan: be sure you have also set expiration for deleted ans spam messages.