Similarly, we wanted to allow people to respond to tickets by "replying" to ticket emails, but did not want new tickets automatically created if a new email was sent to default address. Mainly we were worried about spam creating tickets.
What we're liking so far is using an "Email Filter" in cPanel to forward emails that do not e.g. have a ticket #id to another email address. Then we can just have one agent monitor and deal with inquiries or spam that come in that way. I would guess many other systems have similar email filter functionality.