I thought about doing flask, because I love Python and it would be way easier than monkeying around with Angular. But most of my code is D3, so it's a lot of javascript anyway. And by doing angular, I basically upload one page to the user, and then from there out, they never touch my server. (Data is served by Cloudant - a YC comp). So even though I've had > 10k page views, my server is hardly touched. And everyone gets really responsive interaction since it is all done in the browser, occasionally waiting for new data to serve. So, while this was painful to build, I can really see the allure of client-side single-page apps.
