I really don't even tend to fill out a full stack, I rarely get that much crystal. I use summons before tough autobattles so they get killed instead of a valuable unit, since the autobattle is horrible and only the last unit in a stack ever takes damage. The earth elemental is always useful in that way, even late game. But ya, I think other than as damage-sinks they tend to suck, maybe rather than giving them special army slots just have most units without weapons (monsters, summons, beasts, whatever) gain damage every level, and give the single units more health per level, really make the summoner traits useful, and a reason to keep tamed beasts around for a while.
