It was not uncommon for wealthy folk to carry servants into a war, they would to cook, clean and stay around camp.
Hell, some people would bring their wives into a campaign.
Looking at the raws, she says "フフフ、力仕事がお得意の庶民の方ならきっと平気な顔で戻ってくるでしょ?" in the bottom left bubble of the first page (when she says that physical labor is a commoner's specialty). I think she is actually referring to her master as "Mr Commoner" and essentially saying: "I'm sure that Mr Commoner whose strong suit is physical labor will return with a cool/nonchalant expression on his face". She seems to be more affectionate towards him than in the translation.
Slaves were usually private properties. Taking them to wars would piss off the nobles. If the slaves accompany the nobles, they only did slave duties and took care of the nobles. There were some that were ordered to the battlefield by the noble owners, but this was usually considered as un-noblelike. Most slave soldiers were usually those captured in battles, and were bought directly from the slave traders.
No one brings a maid or a butler in a battle after all. That is because, they need to do their job of housekeeping. More of common sense rather than feeling.