I did, Bill. But as I hadn't declared it at the start of the game, it seemed underhand to suddenly introduce it when my camp was threatened. And, visually, it wouldn't have looked that good balanced on the ridge of the tent I was using as my camp.
On the other hand, Terry's army required so many Psiloi that I gather he didn't have any spares.
I know it's what the rules say. Just strikes me as a bit of an nonsense.