For example, can you imagine such unit as naked fanatics forming testudo formation? Or german night raiders forming phalanx or wedge formations?
No because these units never existed... thus they can't form any formations. Special abiltities are just plain dumb when you have to click on them to activate it. Give warbands some moral bonus, and mabye make the Romans afraid of them, but don't make me click on it.
edit:About those naked fanatics, I'm not saying no Celt, Gaul or German fought naked, but certaintly not enough to make a unit out of it.