I wouldn't consider it Unsound football. I'm not trying to work out a glitch, like you said a 3-3-5 does the same thing, and the Computer (Steelers have used it a lot) uses it on me, so I made my own version. The 3-3-5 doesn't work for my team for some reason, so I use 9 in the box. It is beatable by a lot of teams, but I think it's effective for my personnel so why wouldn't you do what works for your team? When it's not working I change out for a more traditional 4-3, but more often then not it works. Great coaches find systems that work for them, and fit personnel to it. I've fit a system to my personnel, and will continue to find players that fit it.
I have fast linebackers, so I blitz them often, they can't guard a 10 year old girl in man or zone coverage, so I take advantage of their strengths. My CBs are great, so I let them play 1 on 1. I'll take my chances with them. My DLinemen can't seem to stay on their feet so I put 3 pancakes on the field instead of 4.