What kind of magic do you like the most?

1. The larger the army, the less likely it is that a Phoenix/Elemental will tip the scales in your favor.

This is only true in single-player. In online games, battles rarely last that long. Unless it's agreed upon beforehand...

2. If a hero has the "Deadly Chill" skill, and the army includes Messengers or Pit Lords, then summoning a Phoenix is highly discouraged.

Yes, that's true. Although Messengers and Pit Lords don't have very high speed or initiative, so the Phoenix still needs to catch up to them. But Chaos magic from the enemy is a complete disaster for the Phoenix.

4. There's this "Ring of Banishment," it's inexpensive, appears quite often in the shop, and might even be given at the start. If your opponent has it, it's better not to invest in summoning.

The Ring of Banishment doesn't save you from the Phoenix in version 3.1, it's been tested. It deals about 200 or 300 damage, but the Phoenix only has 800 or 900 hit points, for example (although it's often more). It can be defeated in three or four hits, but it will have time to cause a lot of damage and summon another one, as nothing prevents it.
So, the only drawback is number 2. But Darkness has other disadvantages that haven't been mentioned:
1) It's not very effective against the Academy. Titans, Taburetkas, Golems – no mind control spells. Djinns have resistance. The hero has a Magic Mirror.
2) It's not very effective against Elves. Elves often develop Light magic and counter it, plus Unicorns with an aura, plus they can take "Magic Resistance," which is almost always a priority.
So, the disadvantages are much simpler here.

Well, choosing only summoning as a Dark Knight's skill seems like a questionable decision to me.

Don't get too carried away with magic schools. Of course, Chaos magic is useful for a Dark Knight in the early game, but spending two skill slots on magic can be dangerous. I would rather develop a secondary hero with Chaos magic...