Why are Larry, Moe and Curly hidden nationality units? They come so early, and have so little difficulty destroying scouts and disciples, that open borders are a sure way to level them quickly and painlessly. (Not that this isn't fun...

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I guess it depends on your game speed. I play Epic, and unless you really rush Cartography and Pact of the Nilhorn, they don't come that early. One thing that helps me get the Stooges is that I find the AI civs don't make the Pact a priority.
You are right that they can mow down scouts, disciples, and warriors, not to mention barb units with the Orcish promotion. However, once the Hunters, Lizardmen, Satyrs, etc. start appearing things are a little tougher for the Giants. Then, when Tier III units show up, they are really relegated to bombardment duty.
The other thing is with that HN, I find it takes a very long time for damaged Stooges to heal, especially in enemy territory. A lone, healing Curley is dead meat for just about any wandering unit. So, you must send them out in a group, and, even then, a few hunters can make short work of them. It is nice if you can get Council of Esus a produce a priest unit with Medic II to accompany them inside AI civs' borders - healing is much faster then.
I agree with you in that you can have some fun with them and get some gold with their pillaging too.
