With the large box flexibility you get with webdev, I don't know why you would want to use it except for specialized parts. Like a RAW -> jpg transcoder.
- Performance is important even for web applications. For example, after migrating to HHVM, Wikipedia could buy less servers, serve pages faster, and reduce latency.[1] Rust is fast, even compared to other statically compiled languages.
- Safety is also good to have for web development. I personally find it difficult to refactor my Python web applications. This isn't necessarily an advantage of Rust solely, statically typed languages tend to have this property more or less. But Rust does provide more guarantees than ordinary languages do.
[1] http://blog.wikimedia.org/2014/12/29/how-we-made-editing-wik...
A reason why I would choose Rust possibly is because you can prevent data races although with it's memory management model.