The extension is unnecessary for syntax highlighting. Shebang lines are supposed to be used for detecting the file type.

As for knowing the file type at a glance, I'm not sure how often I need this. I'm normally looking for the file by name anyway. If I needed to determine the file-type, I'd write a script to parse the shebang lines of executable files in the current directory and generate a list of the files with their hypothetical extensions (based on a hash/dictionary/whatever). I don't need that very often, so, for me, the trade is worth it.