I love the idea of the Pack Mule - goes back to the days of oldschool D&D when players would hire men-at-arms, linkboys, and porters to round out their adventuring bands. However, I am failing to see the point of having one. A wizard already has five item slots and an apprentice four. Are you likely to accumulate more stuff than this that you haven't parceled out to other soldiers?
I haven't played the game yet, but when we start our campaign, I'm not sure we'll manage more than once a month. An average game would net you three treasure tokens - 40% of these are Grimoires which you aren't going to carry around, 20% are consumables that you will probably use during later games so they aren't going to stack up long, and 15% are gold coins - some of which are going to go to replace soldiers. I just can't see accumulating so much stuff that I need a guy following me around with three more items.
Can a pack mule carry more than one treasure? It wouldn't seem so as carrying treasure does not preclude other soldiers from carrying their one item. Maybe they should be able to carry their treasure unemcumbered?
I don't know... again, I love the concept so if I am missing something someone set me straight or I'll just fill that last slot with another Thug. I can always hire a Pack Mule later when my mighty wizard teeters under his magic arsenal!