Marking a mail as junk simply redirects it to the junk box. Helps reduce crap the user sees but doesn't actually change the landscape. Overtime, junk folder traffic may exceed inbox. My 10+ years old Hotmail and yahoo accounts are already there - daily incoming rate to junk folder is higher than my inbox.
If Exchange could reject (not bounce) mails from explicitly junked senders as invalid email ID (511?), I suspect it would slow things down at least a little.
This should be tracked at the user level so one user cannot junk a legitimate sender for the entire org.