Not at all. Even when you are serving up an "application", sending a broken page isn't a good idea. I've written a couple very heavyweight "web apps" myself (for in-house use) and even those are careful to always send full pages even though page-rewriting was used most of the time. Given that this was easy to do in rails, I fail to see why rendering the template on the server during the first request is hard. It's pure lazyness by these newer frameworks.

If "app" means some pure-javascript game or similar, the very least you can do is provide a proper page that indicates that the game requires javascript. A warning message conveys useful information - a borken template or empty body tag conveys "bugged website".