Well, I somewhat disagree with the software firewall comment. They are helpful if you want to prevent a certain app (not even a virus or something, but let's just say for example, I don't want Office 2007 connecting to the Internet), then I can deny it that ability with the software firewall. You can probably do this with a built-in hardware firewall (in the router) but it's more complicated.
Me personally, I like having both (the software and hardware firewalls between the world and my PC's data). Good in case something happens to one of them. Nowu sing P2P networks can be tricky at times, but to me, it's worth the extra hassle for the added sense of security.
I could see if you have multiple firewalls running on one PC... that's a problem and a no-no in computer land (unless you have very good reason to do it). I always tell my clients one software firewall and one hardware firewall (especially if you use broadband/DSL). Simply because from what I know) not all DSL modems/cable modems have built-in firewalls or any type of protection for protecting the computer connected to it.
Okay... I come across this argument a lot from my peers and I can't refute what you are saying except I get better results with running Malwarebytes' Anti-Malware in the background if I am concerned where I am going on the internet.
I run 15 computers in my business and software firewalls are one big pain in the arse. With Windows 7 at least I can open up the LAN and keep "Public" fire walled. That works for me.
Lots and lots of customers do not have anywhere near our expertise or common sense and again Malwarebytes' Anti-Malware running in the background keeps them from getting nailed most times. I know it works because if it didn't I would lose the customers. Customer satisfaction and service has got to be the highest priority over anything.
When, once-apon -a-time, I left the software firewalls on; I always got the calls at the strangest hours and even late Sunday nights "I can't get here or I can't get there". Bad business when the customer gets frustrated.
I find a way to protect them that works for the both of us. Win-Win.
But again... I am not really arguing your position on software firewalls. I just think Malwarebytes' Anti-Malware is a better solution.