Hamano replied pointing me to this thread. https://public-inbox.org/git/Pine.LNX.4.64.0510171814430.336... Linus proposed adding the feature to git-checkout, subsuming git-checkout-index. Junio expressed concern about file/branch ambiguity; Linus wrote back, "Yes, I know it's ambigious at times, but it really is very convenient. Usually we allow a "--" to say where a filename starts when it is ambiguous."

I think a separate command would have been convenient, too. ¯\_(ツ)_/¯