Xaric- that's right. If he's playing just Nagash and summoning a horde of zombies, he's betting on the two of you finishing the game and not running out of time.
Each Lord of Change has a couple special rules that make it pretty easy for them to summon themselves:
1- They add +1 to the spell rolls of all Tzeentch wizards. The rules don't say anything about abilities not stacking, so if you have two or three Lords of Change out, you get +2 or +3 (Kairos gets +2 from a different ability of his, so if you combo them, you get plus a lot).
2- They have another ability that when the roll to cast a spell, they can make the lower die match the higher one. So if you rolled a five and a two, you just rolled a ten. With only the initial plus one, they need to roll two dice, and one or the other needs to be at least four. As you get more Lords out on the field, the number that you need to roll drops.
So, what started out as a very difficult summoning roll (9+) is way more likely to succeed than to fail (4+ on either die). If Fateweaver is around, have him go first, because he'll only need a 3+ on either die.
Now, I've honestly always been a big fan of the Lord of Change- he might be my favorite GW character/species ever, so the list actually does appeal a bit to me (I imagine making about four or five of them, each converted to be wholly individual). The number you need isn't infinite, you only need enough that they can't be tabled in one turn (because whoever's still around gets to summon the rest of them back).
And unlike the Tomb Scorpion or Fate-Bell-Cheat, you still get to play the game.