Extremely high Brave and extremely low Faith aren't a problem. It's low Brave and high Faith that will make units leave your party; don't let anyone's permanent Brave go below 6, or permanent Faith go above 94.

The Thief can profit from high Brave, just as any one does. A higher Brave stat means a higher chance for your reaction ability to work. Low Brave is only necessary for the party's Move-Find Itemer. Everyone else should have high Brave.