Since I had a similar idea to build something like this, here's my opinion of the pros of this concept:

1. We web developers should always be building to a common specification, and not to the quirks of a particular web engine (which now includes webview quirks too).

2. Javascript is a shitty language. There are other better languages for application development, which allow you harness the full power of the OS platform.

3. It's more in line with unix philosophy - Write programs that do one thing and do it well, write programs to work together and write programs to handle text streams, because that is a universal interface.

4. If an application is built with point 3 in mind, a lot of old codes can be reused and / or given a new UI with this concept.

5. Really tiny as it doesn't increase the size of the application code base by much.