For other factions, chaos magic becomes useless towards the end, as all heroes have artifacts that provide magic defense and resistance. But for Warlocks, I would take chaos magic. If you find a pendant of skill, it will be very effective towards the end. And in long games, the entire map is usually revealed, which means there will be frequent skirmishes and battles, and this means that heroes will not have over 9000 units, so, in my opinion, chaos magic is worth taking for a Dark Elf.
Regarding the topic - summoning is powerful. It's great to use archers as a wall of swords to protect against large creatures.