I don't know where you get the "It's getting less and less useful...." from, if it stops just one malicious unauthorised outbound connection it has just saved your ass, so I would say it is very useful.
Any malware that manages to get past your defences will have free reign to connect to the internet to either download more of the same, pass your personal data (sensitive or otherwise, user names, passwords, keylogger retrieved data, etc.) or open a backdoor to your computer, so outbound protection is essential.
So any zero day or hidden infection could get past your AV so the next line of defence is your software firewall. The more layers you have to your defence the less likely it is to be breached.