They weren't "expendable" in any sense beyond the sense in which any other soldier is considered "expendable." Mercenaries were expected to fight as ordinary soldiers would, but no commander would bother to pay for men that he was planning on eliminating. Many people, possibly because of the popularity of Machiavelli, have the idea of "mercenaries" in the middle ages as a very distinct class from regular soldiers. There is (in theory) distinct line between uniformed troops of nation-states and hired guns. That's not the case with medieval armies. Entire books have been filled with attempts to clarify what is and isn't a mercenary. Are paid professionals like siege engineers mercenaries? Are foreign allies providing soldiers for hire (like English archers fighting for Portuguese kings, or Scottish troops in France) really just making mercenaries available? The term "mercenary" seems to be defined by Supreme Court Justice Potter Stewart's famous phrase: "I know it when I see it." This is all a bit besides the point, but the TL;DR is that mercenary troops (as we might call certain types of soldiers today) were not necessarily defined or seen as such by their medieval employers and weren't as a rule differentiated from other soldiers.