This is the first major redesign in 10 years, I'd say they aren't doing it just to look hip. Additionally from the few screenshots I have seen, it doesn't look like even the most casual of users will have to re-learn everything.
I agree with your argument that redesigns should have a purpose, but I disagree that the argument can be applied here.
They weren't doing it for 10 years because they were terrified of becoming digg and giving users a major gripe. They're already inserting a lot more ads into the feeds, so they're on pretty shaky ground.
Last I checked reddit's reasoning for the redesign was that their current codebase was difficult to work with due to "the lack of separation of the frontend and backend code"
I agree with your argument that redesigns should have a purpose, but I disagree that the argument can be applied here.