Vim does handle this. Here is Vim (and I've removed my .vimrc file, so this is not a setting I have set personally), on a Makefile where the third line erroneously uses spaces:

https://i.imgur.com/xMHXeL4.png

> I just don't see how insisting on tab and not space is remotely defensible.

This is like insisting that people haven't been having religious wars over this sort topic for years. You can see the Wiki[1] page on it (which starts "This is one of the eternal Holy Wars") for arguments as to why someone might prefer tabs. I personally like them because they allow the reader of the code to choose the visual layout of the indentation.

Now, while I love tabs, I work in Python all day. But good editors, like Vim, allow you to customize the indentation settings sufficiently to handle both cases like Python (spaces for indent) and Makefile (tabs) gracefully. So "Indent" and "Dedent" just do The Right Thing™ and it mostly never matters.

[1]: http://wiki.c2.com/?TabsVersusSpaces