The proposed tab → 4_space preprocess is lossy, and specifically drops tab-nesting information that is usable for the kind of plain text outlining that can be done in @foldingtext, for example, and which seems likely to become an important use of MD.
The Gruber spec clearly doesn’t anticipate outlining, and creates an ambiguous frontier between code lines and tab-nested list or body lines.
MD document as outline (any node with hash-defined, or tab-indent-defined children can be collapsed, hoisted, moved with children etc in @foldingtext) is clearly an advance, and one which is dependent on preserving a distinction between tabs and spaces.
The destructive tab to space preprocess seems retrogressive, and parts company with plain text outlining.
If this spec is going to become any more than a MacFarlaneMarkDown™ niche, I think the conflation of tabs with spaces might be worth reviewing.
Rob