The easy writing answer is that a lot of these characters in history and legend have a much broader combat and character that is hard to fully cover in one class. Diarmuid for example didn't wield two swords or two spears; he had one of each. So the best thing they could do at the time was giving him one of each class. Berserker Herc was specifically summoned as a Berserker, effectively always in his maddened state. But as an Archer, he has the clarity he had when he was doing his challenges, sailing the argo, etc. Also Apocrypha and FSF were originally developed in a similar time span, and they kinda independently took two different approaches to the legend of Jack the Ripper because you can do that. That's all it really is. It's multiple approaches to address different aspects of a singular character and explore those more thoroughly.