I would have always said the same thing. But my brother bought his new wife a 17K ring when they were both barely out of grad school, 200K in debt, living with my parents, and he financed over half that. I thought they were nuts, and I was secretly damn judgmental about it. But they are both ridiculously happy with their decision, and she'll have a ring for life that she loves and is stunning. Their financial situation will improve-- school debt isn't for nothing-- and this debt will be a blip in their lives.
If you know you're income is on the way up, and you have a plan, there's nothing wrong with financing a ring, unless you're uncomfortable with it. It's just not true that you can't afford it if you don't have the cash. I have completely changed my view on this.