Resources need to be able to send and receive mails because that's how they receive and accept invitations.
I agree that they're like a backdoor account, but maybe Zimbra NE has some type of limitation on them (such as X resources per Y user account licenses)--however if there is a limitation, I haven't seen it documented.
The idea of using a resource to hold public folders is a great idea--it's a workaround for the problem posed by bugs
15855 and
18837.
Aside from that, there may be some other hidden limitations, as well as the general "ugliness" of using resources as regular accounts, which would probably inhibit NE customers from trying to cheat Zimbra, if the ethics of a business relationship aren't enough.
EDIT: I suppose that Zimbra could make it impossible to log into a resource account other than through the Admin GUI "View Mail" option.