>>2018079
Depends on the summon. The evolving ones you ideally want to keep alive because it's just great value if you get the evolved version, but the value did get diminished quite a bit when they reworked the experience ranks, because going from a max XP unit to a zero XP unit you lose a lot more now, especially HP. The evolved version is still better but not by as much as it used to be. Also, some of the high-tier summons cost a lot of casting points so you can't always summon them instantly.

But in general yeah, summons are intended more expendable than recruits, since you can just summon more of them while you're on the go while you can only get recruited units from your cities which often isn't practical when you're on the offense.