well, think, being a summoner, if you have a powerful incarnate against a strong boss, would you throw it at them so that they can beat the shit out of it and you spend the next 5 turns flailing at them with your feeble attacks then get your face stomped? no. You use your secondary summon to wear down the enemy, wait for favorable elemental surface and position, and then drop your incarnate, so that at least before it dies it can use all of its abilities, this situation happens a lot when playing lone wolf
However I'm ok with adding summons from other branches, makes no difference to me tbh