Windows firewall may be strong enough, I will not say more then strong enough. It is no doubt a decent firewall. But based on your statements MSE only plays well with built in windows features. I prefer Avast to MSE as it has served me well. I also prefer Comodo firewall with Defense + turned on. These play well with each other, and with real time Malwarebytes and Superantispyware.
I personally have no system stability issues, as is true of many others. So I also will not agree that it will vastly improve system stability. IF one has issues with their chosen AV/Malware programs causing system instability, then certainly MSE and the windows firewall may well be a good alternative, but it doesn't mean it is the better choice for everyone.
People use Norton with no issues, and are happy with it. I'm sure the same can be said of McAfee (I'm pushing it there
). For them, what works is the best choice. A Guy