Sorry to step on your toes.
I felt since she was talking about a router, and her question was about a physical firewall. By default a firewalls purpose is to stop incoming traffic and allowing all out going traffic. We could argue all day about fire walls and both of us would be right.
It comes down to the level of security vs. usablitly. She could lock down her system with so much security she would feel constipated. But, as you so graciously pointed out most of the readers dont have the knowledge to manipulate around all of that security! Sure, you and I could but I am not sure about 90% of the population. So what do you do!
I am of the opinion that one, The only need for a firewall is to stop incoming Hackers traffic. Two, Hackers arent thinking of Joe Blow's network of Mom, Dad and Johnny! For the inexperienced user a firewall can be a hugh hassel, when it comes to connecting your computers, printers and creating backups at home.
So here we stand two gun slinger in the wild west!
I feel for a home user it is either /or and I choose the hardware before the desktop software!Block it before they get into your computer.
Would you rather stop a burglar before they break in to your home or stop them while they are standing over you in your bedroom!
ps.-its "plain" irresponsible! Brrrrrrrrrrrrrrrrrrr