@Rubber: Joining the fight less often than Germanics implies that the civilized generals had more pressing concerns than leading his soldiers, such as observing the wider battle and making tactical adjustments on the go as circumstances deviate from his plans and expectations.
The civilized general differs from an average barbarian warlord in his battlefield role: deploying his troops in favorable positions, keeping and committing reserves when necessary, getting closer to the crucial point of action to inspire his men, and sometimes even personally leading his elite guard into the hottest killing zones and set a strong example, like Alexander loved to do.
The barbarian warlord lived and died by the ethors of "lead by example", and very few of them commanded a battle like their so-called civilized counterparts, most preferring to lead their personal retainers first into the fray - one of the reasons they like to keep large bodies of household warriors. Tactics and strategical thinking more often than not escaped any Germanic warchief's head once the fighting started and the blood-lust overcame his senses.
Anyway, I'm missing the point. Fighting more often did call for the need of larger warbands, but I doubt civilized generals would gather smaller bands since they too had to commit themselves to the hot spots at critical moments. By 500AD, the barbarians had evolved so much from constant military and political contact with Rome that it changed their war-making ways forever, and the importance of tactical decisions slowly began to take root in the heads of Germanics again, as shown by their growing success against the legions. Hence I believe basic statistical variations between civilized and barbarian bodyguards should do the trick.
@eth: I really have little idea concerning the protection ancient generals had against arrow showers. I think it would depend on the power of the bows used and arrow designs. e.g. English Longbows. Pretty much like how any soldier would face an arrow shower, really, with his shield, his armour, his helmet and luck. IMHO, the present state is fine.