They're great for killing generals. In my WRE campaign, Placus the Horseman (Dux of Britannia) was killed almost instantly in a 2000 man battle by a unit of Celtic berserkers. My army routed instantly since the apposing Celtic king had excellent stats. I almost ended up losing Britain thanks to that battle. It was also the first time I experienced a “crushing defeat” in a land battle at the hands of the AI.