In TeX, the difference is that an EM QUAD (\qquad) and an EN QUAD
(\quad) provide spaces that are legitimate breakpoints for lines within a
paragraph; while EM SPACE, EN SPACE (\enspace) and THIN SPACE (\thinspace)
produce horizontal space that cannot cause a line-break.
My assumption on reading the Unicode standard was that this was the
intention---though it is not spelled out anywhere. Maybe a clarification
would be worthwhile. If so, the fact that TeX is so widely implemented and
used---at least within the arena of technical documents---might make it
worthwhile to preserve those characteristics.
o o o (_|/
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