It's kinda hard to grasp. I kinda like the brave/default system because it gives more value to defending.
As long as you can juggle between defaults and braves well enough there should be no problem.

I've found that in boss battles I let my most important team members (healers and cannons) default for the first three rounds, then pelt the boss with everything I've got. Sometimes I might have EVERYONE default the first three rounds.
So you don't have to render your characters useless as long as you play it safe. It's all up to strategy.