All the JavaScript and templating code should be reusable by other platforms with no effort. It's required for testing, optimization and being able to move from one technology to other easily. This is the challenge programmers should solve. There has been already dozens of very good client-side frameworks that let us create very cool UIs with no effort but none of them is still good enough since the core idea is bad.
While you have a point, you seem to miss the original problem of browser frameworks. Browsers work _differently_. This is not the problem of a framework. Moreover nothing can stop you from using node.js + DOM mock to test anything you want. Unfortunately most of these tests will stay useless. So it's not about a possibility to write them. It's about a sense.
And btw, we are going to implement node.js tests to use Travis very soon.
