I agree this would be great. However, it should also stop you from providing useless tarballs (as `/archive/` does today) if:
- you use autoconf (or any other tool(s) that require generating code into the source archive; or
- you have submodules (to which `git archive` is completely blind).
Note that `git-archive-all`[1] can help as long as your submodules don't do things like `[attr]custom-attr` in their `.gitattributes` as it is only allowed in the top-level `.gitattributes` file and cannot be added to the tree otherwise.
[1]https://github.com/roehling/git-archive-all