The fact that a random commenter on HN used a non-monospaced font with refterm actually makes it a use case.
I do, however, agree that it is an edge case with a very low probability.
The Microsoft terminal doesn't render monospaced fonts, the overwhelmingly common case, nearly as fast as refterm. If rendering variable-width fonts is somehow intrinsically insanely expensive for some reason (which I haven't seen anyone provide good evidence for), then a good implementation would still just take refterm's fast monospaced rendering implementation and use it for monospaced fonts, and a slower implementation for variable-width fonts.
That is - refterm's non-existent variable-width font rendering capabilities do not excuse the Windows terminal's abysmal fixed-width font rendering capabilities.
This is an edge case as much as building a rasterizer directly into the terminal is an "edge case".