...for the event that APU bleed is introduced at some later point (bleed failure? After shutdown?) which would OPEN the XBLEED if it was left in AUTO.
As for opening it if no eng fire, you can run both packs from 1 eng bleed (though not recommended at high altitude, in my experience), so perhaps as well as the reasons you suggest, it is to improve cabin ventilation and/or pressurisation. Just a guess.