Darkness is unreliable due to its high resistance, and summoning loses its effectiveness in the late game. Therefore, it should be countered with light, and all that remains is to acquire the spells.

And when you already have huge armies after several months on a huge map, what good is summoning anymore, besides raising the dead? The Phoenix is practically useless (even if it has 1000 health, it will still be taken down in 1-2 hits), unless you summon corresponding elementals with Fire Warriors, but they only appear every 10 turns, figuratively speaking. So yes, the point about summoning on very large maps is ambiguous.
By the way, if the enemy has Banish, does it affect the summoned phoenix? I don't think I've encountered that before.