Good single-page app development is just as clean (I've done both), and will be faster over the long term, and scales out better. It keeps all the components cacheable and DRY. The benefits are all about data locality. If your code and data are cached on the client, then the only the minimal subset that has changed needs to be invalidated and updated. If it's only data that has changed, that's all that's retrieved. If it's only code, same goes here. For old-school MVC, if a single character has changed, the whole page is invalidated and must be re-rendered.
The tangible benefits when done right (there's lots of crappy code in every language and system) are greatly reduced user-perceived latency (the system reacts immediately, indicating that something is currently being updated, etc).
The tangible benefits when done right (there's lots of crappy code in every language and system) are greatly reduced user-perceived latency (the system reacts immediately, indicating that something is currently being updated, etc).