From what I've read, if you skip cavern layer 3, you miss out on all the stuff that comes in cavern layer 3, like nether-caps.
OK, I stand corrected:
http://df.magmawiki.com/index.php/Cavern#VegetationRemoving a layer will cause the layer above to randomly pick from trees that it could have handled and the layer above can handle
So I guess that if we nixed the third cavern layer, the second would probably contain the nether-caps and blood thorns it was supposed to have.
I still don't see any particular reason to nix the third cavern layer.
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Timothy Collett
"The only thing you can't trade for your heart's desire...is your heart." "You are what you do. Choose again, and change." "One of these days, someone's gonna plug you, and you're going to die saying, 'What did I say? What did I say?'" ~ Miles Naismith Vorkosigan
