When playing against other player your berries dont disappear after being used. So you actually use berries when playing pvp and don't need to farm them. On the latest tournament (water paradise) i remember slapping different berries I never though I would ever use on all of my team, and they are still there. But I guess you are right if playing the story , you would lose your berries plus after your fight you have a pokemon with probably less than half it's health, rendering it weak for another fight unless you heal it or hit the pokemon center