Poison ivy berries are eaten by songbirds in the winter. To me, that's enough to justify its existence and actually make me like the plant.
I have this vision of one day protecting my yard by shrubs and cedar trees inundated with poison ivy. I always figured I'd be able to point out the criminal who broke into my house and stole my lousy stereo system in the line-up.
"Yes, officer, he's the second one there. The one covered in red bumps that is itching really badly."
If that doesn't convince you to keep it around... my cousin says: To kill poison ivy you can use straight white vinegar (it will pretty much kill any plant), only humans are allergic to it and goats love to eat it.