Zombie Warriors teach you to to build your commons as Magic. The only way for Infect to trigger is if you have a Zombie in your own discard pile, and the main two ways for one to get there is if you Magic Drain them from an opponent who killed one, or if you build them as Magic.
The other way for Zombies to get to your magic pile is if you kill them with your own attacks. That's right, destroying your own Units gives you magic, which is another important lesson.
Dark Sacrifice pushes this idea even further. Not only can you destroy your own Units as Magic, you can turn them into healing for your own important Champion!
Elut Bal takes it one more step and says "No, seriously, discard some of those commons and get this Champion out. Trust me!"
And if you still can't afford them, there's always Force Summon which weakens
Ret-Talus and gets a Champ out for cheap. It's not always worth the risk against some speedy and tricky factions, like the
Cloaks, Jungle or
Shadow Elves.
Now you might be thinking, "but James, I summon Commons all the time with the
Fallen Kingdom.
Legions of the Dead let's me get three guys out for half their usual price and out of the discard pile to boot. That's wicked."
And you'd be right. The
Fallen Kingdom let you have your decadent cake and eat it too, but at a price. Their Summoner is weak in combat, and most of their Units have pretty low attack values and short threat ranges. But if you're reluctant to get Champions out, they make it too easy to pass up.