The AI doesn't usually use eternity unless it has a few extra time quanta beyond 3, like 4 or 5. Also, certain creatures are considered 'too crappy' to rewind. For example, I was playing a game yesterday where I had a bunch of stupid creatures from shard of serendipity, like graviton guard, schrodinger's cat, etc, and the AI had a bunch of rewinds in hand (precognition showed me them) but wasn't using them until I drew some better creatures, and then it started spamming them. Since dune scorpion is fairly small in stats, the AI probably thought it was 'too crappy' to bother with. I'm sure there's a formula for the AI to determine which creatures are the biggest threat, and whether it is worth rewinding something. Players do the same thing, too; if you know/suspect your opponent has dragons in their deck, and they start with photons, you might often wait before using eternity/rewind. The AI just isn't as good at it :p