I checked Unnerve's page but it doesn't show something except being angry to eat berries. So why would anyone use that ability?
It stops your opponent from being able to eat berries. For example if you had a galvantula with unnerve vs another Pokemon holding a sitrus berry the berry that would usually trigger at 30% health won't be eaten. In terms of viability I wouldn't say it's very good as an ability though.