Have you used the beta version of WordPress 4.9? I have. One of the new things is the file editor, which has been pretty fully overhauled.
It has a lot of improvements. If we are to have a file editor at all, these are proper changes. I joked about WordPress with an IDE on Twitter, and Ryan Sullivan smartly reminded me of my own position from a comment on his blog.
I had some long discussion / debate with Otto about this once. I agree with you that it’s better off just not being there, but he had some decent reasons about why I was wrong.
However, we did agree on something: there is room for some built in syntax checking and perhaps some safety precautions that would prevent, say, someone from completely borking their site. I think the struggle is that this is a) an easy thing to take out b) a difficult thing to do *well* and leave in c) even easier to just ignore.
Most people hyper-alert to the WordPress world know not to screw with this editor. Out of sight, out of mind. However, that probably doesn’t reflect the true state of how this editor plays in day to day users: a big source for hacks and fails.
Anyway, let's put aside the argument over the file editor existing. Weston brought up a very good point, that I think should be the primary benefit for this new syntax checking system: it will be used in text widgets and custom CSS windows.
This is overwhelmingly a good thing. In fact, I wonder if there's a case for enabling HTML syntax checking in the text-view of the post editor as well.
Anyway, I'm now much more satisfied with this project that I was before. Also, the warning system that's gone in the file editor is a big improvement too. Still, as Scott Taylor noted, for actual file editing, this should be considered the crudest of options, not a good practice.