>>89698346DS (and ER) have a somewhat schizophrenic approach to summons, because some are used to progress character questlines and some are not
there are also speculations and lore reasons behind certain characters being summonable in specific fights, so lorefags like Mori can be talked into using those on that basis
all in all, theoretically if you want to fully experience the story of the game, summoning is encouraged but it's discouraged from gameplay perspective since it tends to mess with boss AI/aggro and end in fights where the boss is reduced to a punching bag
