After reading Relationship between _index.md and index.html my take away is that you use _index.md if you have front matter. But in my experience any markdown file can have markdown?
IIUC _index.md is only useful… I think, for list pages?
You do not have to create an _index.md file for every list page (i.e. section, taxonomy, taxonomy terms, etc) or the homepage. If Hugo does not find an _index.md within the respective content section when rendering a list template, the page will be created but with no {{.Content}} and only the default values for .Title etc.
Well there must be a lot of pages like mine that sort of mix _index.md (list) and actual index.md style content. I guess there is no harm, other than confusion.