> If you write bad code in jQuery you're going to write bad code in a framework.

Preventing this is precisely what frameworks are for. A framework - a good one at least - provides a frame that has you code along guidelines (best practices, opinionated style devised by the framework creator etc.) and therefore prevents you from committing common mistakes.

Using a framework allows you to delegate low-level decisions to someone else, who - hopefully - is more knowledgeable than yourself in that area.