What's the point?
Yes I know I can only summon one at the time (Ghost, Demon or Undead), but when my Demon is killed or his time of summoning is over, I can immediately summon my Ghost (or Undead)
And always my creature + all the enemies I can Charm,
all together it form a nice "fiesta of allies"
Plus: the Ghost does something no other can - besides Charmed 'Healers', but not always there to charm -
Healing my hero ... an effective potions saver .