One benefit worth calling out: the box-drawing fallback is worse than it looks for CJK users. U+2500-257F and the half/full block elements are East Asian "Ambiguous" in UAX #11, so in a terminal configured to render ambiguous as double width (xterm's cjkWidth, iTerm2's checkbox) every line glyph takes two cells and the whole frame lands in the wrong place. Cell borders aren't text, so that failure class just disappears.

The part I'd want specced early: borders are per-cell, but cells aren't per-grapheme. A CJK char owns two cells plus a continuation placeholder, and combining marks or ZWJ emoji sequences own zero-or-more depending on whose wcwidth table you ask, so app and terminal routinely disagree across Unicode versions. Today that shows up as a vertical bar one column off; with cell borders it becomes a line that stops short of the corner. Does the border attach to the cell or to the grapheme cluster?