On one hand, yes it is convenient, but on the other it could become a huge mess. It reminds me of Rails 2.x where it became almost impossible to debug, or fix front end code that used rjs or whatever it was called. Because disparate snippets of JS were littered throughout your code base in files that were hard to find.
I’m sure the Phoenix team has put a lot of effort into it, and I truly hope it slaps. I myself am just really hesitant to use it, when CSS files and non colocated JS work just fine. I’ll probably be waiting a couple years before giving it a try