I agree with Longwings frustration. As I work at a Swedish University I'm counted as an official representative of the Swedish government and with that comes tonnes of rules of what I'm allowed to do and what I'm not allowed to do. As all the emails I get are potentially an email to a government representative I'm not allowed to set my email client to automatically delete emails. I am allowed to set it to automatically send it to a separate mailbox (e.g. Junk mailbox) that I "regularly" glance through (quickly) before deleting the spam emails.
The problem is, however, that to make sure that I really see the emails I need to see I can't rely on Apples junk-filter alone, but need the advanced options (e.g. excluding emails from certain addresses). And for some strange reason Apple has decided that if you use the advanced options the "Erase Junk Mail" command is greyed out.
Come on Apple, show us that you care for your customers and solve this issue!