I would certainly contest that outbound firewalls aren't required.
If an outbound connection is made from your system, the corresponding inbound connections sails right past your firewall as essentially it has permission.
You only have to view the viruses and worms forum to see the number of URL:Mal alerts, etc. resulting from outbound connections by your system. And the only thing that stopped them was avast as they were trying to connect to malicious sites.
Any hidden or undetected malware on your system would be able to make out outbound calls to whatever site unmolested. By saying any malware on your system could already be creating holes in your system, well why not just give up. Not all malware will be capable if doing that straight off, a lot (viruses and worms) have a connection to malicious sites to bolster what is already on the system.
EDIT: Not to mention this article was Published 09/17/13, a lot has changed since then, firewall, antivirus and malware since then.