It is definitely a viable league concept, but not always the easiest one.
With a normal league - hopefully you can blunt some of the opponent's offense by thinning out there numbers a bit. Of course, they are doing the same thing to you.
With a mostly defensive league, your opportunities to reduce an opponent's attacks are minimized. So, they will continue to throw everything they can at you over-and-over - without any fear of reprisal. And attrition will eventually take its toll.
Unlike most miniatures games, which still use the old I-go-then-you-go format, one of the best ways to discourage an opponent from shooting at you is your ability to shoot back.
I'm not trying to dissuade you. It is a fun league to play, but it takes a bit of "figuring out".
