DRM does not defeat the purpose of copyright. Both copyright and DRM are just means to erect partial-excludability for an information good to encourage a market economy around the exchange of that good. One is a legal barrier, the other is technological.
We do this with physical goods through laws around theft, and police to enforce those laws. Information goods however, are non-rival, thus aren't subject to theft, just copying. So, generally, society doesn't enforce "excludability breaking" with information as stringently as we would with a rival good. Sometimes it does go completely, and inexcusably overboard: see Aaron Swartz.
We haven't had a lot of time to think through what we really want as a society here. Information economics concepts like transparency, excludability and rivalry are still very new to people. The nature of information goods is not a market economy, it's a gift exchange economy. Yet we've built 300 years of progress on a market economy. So we're evolved to a hybrid of market-gift economies in the information sector, but no one really knows where it will end up in the long run.
Amusingly, the most free country on breaking and redistributing digital content is China. I remember the old saw that Adobe Photoshop used to cost $0 in China, but $300 for the manuals. Almost every other country frowns upon such behaviour and tries in some way to make it illegal.
