I think the main thing about having Fergus carrying a cask is that it explains how he loses his hand -- the soldiers wanted the cask of ale, he wouldn't give it to them, they tried to swipe at it with their sword and caught his hand by accident. However, when you remove the cask as they did in the show, why would the redcoat specifically cut off Fergus's hand? It's not like they caught him thieving. It doesn't really make any sense. Yeah, that particular soldier was quite pissed off with Fergus, as they made clear earlier, but it still doesn't really explain or justify why Fergus's hand was cut off. Why not take him into custody and throw him in jail for a few weeks? Or just beat him really badly?