Yes, you need the cross brace. The bags will "flap" a lot more without it. When loaded or on rough roads, this will break stuff eventually.
Why on earth don't you want to use it?
Because I want to save it for a rainy day....
It goes under the wheel arch and the thing is going to be hammered to hell and back with small stones.....and rust. The thinking is that if I put bigger bags on and really go off road, I can put it on (2 minute job). Meanwhile, I'm using the bike in the city and the most it carries is a few groceries, magazines, coffee pot.
That's IF I get any riding done. It's been raining for a month up here.